"Christians at first were few in number, and held the same opinions; but when they grew to be a great multitude, they were divided and separated, each wishing to have his own individual party: for this was their object from the beginning....being thus separated through their numbers, they confute one another, still having, so to speak, one name in common, if indeed they still retain it. And this is the only thing which they are yet ashamed to abandon, while other matters are determined in different ways by the various sects." (cited in Origen's Against Celsus, 3:10, 3:12)
The objection takes different forms, depending on who's using it and for what purpose. Critics of Christianity will often assert that there are too many differing interpretations of the Bible. God, if He had inspired the Bible, would have done so in such a way as to avoid the level of disagreement that exists over the meaning of the text. Roman Catholics and Eastern Orthodox often suggest that sola scriptura is implausible, because there are too many disagreements over the meaning of scripture.
Though the number of disagreements over Biblical interpretation can be expected to increase over time, as with other historical documents, the existence of a large variety of interpretations predates the Reformation, as Celsus' comments above illustrate. The early Christians sometimes referred to the unity they had with other Christians while making comments somewhat similar to those of Celsus on other occasions, much as Christians today refer to both their unity and their disunity with each other in different contexts. A Roman Catholic or Eastern Orthodox critic of Evangelicalism will often quote what Christians of earlier generations said about their unity with each other without quoting what the same sources said on other occasions about their disunity. Many comments similar to those of Celsus can be found in a source like Basil of Caesarea or John Chrysostom. Eusebius of Caesarea went so far as to partially attribute the Diocletian persecution to God's discipline of the church for its disunity (Church History, 8:1:7-8:2:2).
How can anybody know how much unity a religion, a rule of faith, or some other entity must produce in order to be acceptable? What about the insincerity of many people who profess to be Christians? Or the irresponsibility of some who are more sincere? How much is Christianity, Jesus, Paul, the Bible, or any other such source of controversy responsible for the disagreements that exist, and how much are modern interpreters or other factors responsible?
I close with Origen's response to Celsus nearly two thousand years ago. Notice that he makes no appeal to the unity Christians have on the basis of submission to a Pope. (Origen had no concept of a papacy.) He doesn't argue for something like a worldwide denomination or unity around ecumenical councils. Origen's response to Celsus isn't what we so often hear today from Roman Catholics, Eastern Orthodox, and other critics of Evangelicalism. And he mentions many relevant factors that skeptics of Christianity often ignore or underestimate. His response can be improved upon, and it was written in the context of the third century, not our context, but much of it is applicable to modern sources who come at these issues in a manner similar to Celsus' shallow approach:
And as it is no ground of accusation against philosophy, that there exist Sophists, or Epicureans, or Peripatetics, or any others, whoever they may be, who hold false opinions; so neither is it against genuine Christianity that there are some who corrupt the Gospel histories, and who introduce heresies opposed to the meaning of the doctrine of Jesus....
He says, in addition, that "all the Christians were [initially] of one mind," not observing, even in this particular, that from the beginning there were differences of opinion among believers regarding the meaning of the books held to be divine. At all events, while the apostles were still preaching, and while eyewitnesses of the works of Jesus were still teaching His doctrine, there was no small discussion among the converts from Judaism regarding Gentile believers, on the point whether they ought to observe Jewish customs, or should reject the burden of clean and unclean meats, as not being obligatory on those who had abandoned their ancestral Gentile customs, and had become believers in Jesus. Nay, even in the Epistles of Paul, who was contemporary with those who had seen Jesus, certain particulars are found mentioned as having been the subject of dispute,--viz., respecting the resurrection, and whether it were already past, and the day of the Lord, whether it were nigh at hand or not. Nay, the very exhortation to "avoid profane and vain babblings, and oppositions of science falsely so called: which some professing, have erred concerning the faith," is enough to show that from the very beginning, when, as Celsus imagines, believers were few in number, there were certain doctrines interpreted in different ways.
In the next place, since he reproaches us with the existence of heresies in Christianity as being a ground of accusation against it, saying that "when Christians had greatly increased in numbers, they were divided and split up into factions, each individual desiring to have his own party;" and further, that "being thus separated through their numbers, they confute one another, still having, so to speak, one name in common, if indeed they still retain it. And this is the only thing which they are yet ashamed to abandon, while other matters are determined in different ways by the various sects." In reply to which, we say that heresies of different kinds have never originated from any matter in which the principle involved was not important and beneficial to human life. For since the science of medicine is useful and necessary to the human race, and many are the points of dispute in it respecting the manner of curing bodies, there are found, for this reason, numerous heresies confessedly prevailing in the science of medicine among the Greeks, and also, I suppose, among those barbarous nations who profess to employ medicine. And, again, since philosophy makes a profession of the truth, and promises a knowledge of existing things with a view to the regulation of life, and endeavours to teach what is advantageous to our race, and since the investigation of these matters is attended with great differences of opinion, innumerable heresies have consequently sprung up in philosophy, some of which are more celebrated than others. Even Judaism itself afforded a pretext for the origination of heresies, in the different acceptation accorded to the writings of Moses and those of the prophets. So, then, seeing Christianity appeared an object of veneration to men, not to the more servile class alone, as Celsus supposes, but to many among the Greeks who were devoted to literary pursuits, there necessarily originated heresies,--not at all, however, as the result of faction and strife, but through the earnest desire of many literary men to become acquainted with the doctrines of Christianity. The consequence of which was, that, taking in different acceptations those discourses which were believed by all to be divine, there arose heresies, which received their names from those individuals who admired, indeed, the origin of Christianity, but who were led, in some way or other, by certain plausible reasons, to discordant views. And yet no one would act rationally in avoiding medicine because of its heresies; nor would he who aimed at that which is seemly entertain a hatred of philosophy, and adduce its many heresies as a pretext for his antipathy. And so neither are the sacred books of Moses and the prophets to be condemned on account of the heresies in Judaism.
Now, if these arguments hold good, why should we not defend, in the same way, the existence of heresies in Christianity? And respecting these, Paul appears to me to speak in a very striking manner when he says, "For there must be heresies among you, that they who are approved may be made manifest among you." For as that man is "approved" in medicine who, on account of his experience in various (medical) heresies, and his honest examination of the majority of them, has selected the preferable system,--and as the great proficient in philosophy is he who, after acquainting himself experimentally with the various views, has given in his adhesion to the best,--so I would say that the wisest Christian was he who had carefully studied the heresies both of Judaism and Christianity. Whereas he who finds fault with Christianity because of its heresies would find fault also with the teaching of Socrates, from whose school have issued many others of discordant views. Nay, the opinions of Plato might be chargeable with error, on account of Aristotle's having separated from his school, and founded a new one,--on which subject we have remarked in the preceding book. But it appears to me that Celsus has become acquainted with certain heresies which do not possess even the name of Jesus in common with us. Perhaps he had heard of the sects called Ophites and Cainites, or some others of a similar nature, which had departed in all points from the teaching of Jesus. And yet surely this furnishes no ground for a charge against the Christian doctrine.
After this he continues: "Their union is the more wonderful, the more it can be shown to be based on no substantial reason. And yet rebellion is a substantial reason, as well as the advantages which accrue from it, and the fear of external enemies. Such are the causes which give stability to their faith." To this we answer, that our union does thus rest upon a reason, or rather not upon a reason, but upon the divine working, so that its commencement was God's teaching men, in the prophetical writings, to expect the advent of Christ, who was to be the Saviour of mankind. For in so far as this point is not really refuted (although it may seem to be by unbelievers), in the same proportion is the doctrine commended as the doctrine of God, and Jesus shown to be the Son of God both before and after His incarnation. I maintain, moreover, that even after His incarnation, He is always found by those who possess the acutest spiritual vision to be most God-like, and to have really come down to us from God, and to have derived His origin or subsequent development not from human wisdom, but from the manifestation of God within Him, who by His manifold wisdom and miracles established Judaism first, and Christianity afterwards; and the assertion that rebellion, and the advantages attending it, were the originating causes of a doctrine which has converted and improved so many men was effectually refuted.
But again, that it is not the fear of external enemies which strengthens our union, is plain from the fact that this cause, by God's will, has already, for a considerable time, ceased to exist. And it is probable that the secure existence, so far as regards the world, enjoyed by believers at present, will come to an end, since those who calumniate Christianity in every way are again attributing the present frequency of rebellion to the multitude of believers, and to their not being persecuted by the authorities as in old times. For we have learned from the Gospel neither to relax our efforts in days of peace, and to give ourselves up to repose, nor, when the world makes war upon us, to become cowards, and apostatize from the love of the God of all things which is in Jesus Christ. And we clearly manifest the illustrious nature of our origin, and do not (as Celsus imagines) conceal it, when we impress upon the minds of our first converts a contempt for idols, and images of all kinds, and, besides this, raise their thoughts from the worship of created things instead of God, and elevate them to the universal Creator; clearly showing Him to be the subject of prophecy, both from the predictions regarding Him--of which there are many--and from those traditions which have been carefully investigated by such as are able intelligently to understand the Gospels, and the declarations of the apostles....
Now, if he imagine that the existence of numerous heresies among the Christians is a ground of accusation against Christianity, why, in a similar way, should it not be a ground of accusation against philosophy, that the various sects of philosophers differ from each other, not on small and indifferent points, but upon those of the highest importance? Nay, medicine also ought to be a subject of attack, on account of its many conflicting schools. Let it be admitted, then, that there are amongst us some who deny that our God is the same as that of the Jews: nevertheless, on that account those are not to be blamed who prove from the same Scriptures that one and the same Deity is the God of the Jews and of the Gentiles alike, as Paul, too, distinctly says, who was a convert from Judaism to Christianity, "I thank my God, whom I serve from my forefathers with a pure conscience." And let it be admitted also, that there is a third class who call certain persons "carnal," and others "spiritual,"--I think he here means the followers of Valentinus,--yet what does this avail against us, who belong to the Church, and who make it an accusation against such as hold that certain natures are saved, and that others perish in consequence of their natural constitution?
And let it be admitted further, that there are some who give themselves out as Gnostics, in the same way as those Epicureans who call themselves philosophers: yet neither will they who annihilate the doctrine of providence be deemed true philosophers, nor those true Christians who introduce monstrous inventions, which are disapproved of by those who are the disciples of Jesus.