A faithful Catholic very suddenly dies while committing a mortal sin (say lying). Assume every other time he has committed a mortal sin he would faithfully go to confession and receive forgiveness. However in this case his death is quite ill-timed. Does he go to heaven or hell? The consensus is he goes directly to hell . . . do not pass go . . . do not receive $200.
No one knows where he would go, first of all, because that is God's determination, not ours. The Catholic Church has not (to my knowledge) even stated that Judas is in hell (or anyone else, except the devil and other fallen angels who are there - or will be - by definition). All we say is that those committing a mortal sin place themselves in danger of hellfire, if it is unrepented-of. That's nothing more than what the Apostle Paul does, in a number of passages (see below).
But you should understand exactly what a mortal sin is, too. It requires three things: 1) grave or serious matter; 2) sufficient reflection; and 3) full consent of the will. In the scenario above one or more of these things may not be present (it may have been a relatively minor "white Lie," etc.), in which case the person would definitely not be damned because of this one thing. There may not have been time enough for the person to be responsible for all these. God knows what the person would have done if he had had more time, and takes that into consideration, I believe (because He knows all things, which includes hypotheticals and all possible future scenarios).
Moreover, we believe that because God is sovereign and ultimately rules over all things in His Providence (Catholics believe this, too) - and above all, merciful -, that He would "arrange" things so that this person was not unjustly judged for a momentary lapse, so to speak. Therefore, it is not as simple as this classic, garden-variety objection to Catholicism would have it.
Here is my section on this topic, from my upcoming book, The One-Minute Apologist (Sophia Institute Press)
There is no such thing as mortal and venial sins
Whoever fails in one sin is guilty of breaking all of the Law (Jas. 2:10)
The Bible plainly teaches that there is such a thing as a mortal sin (1 John 5:16-17), and often refers to lesser and greater sins, thus supporting Catholic theology.
Some non-Catholic Christians think that all sins are exactly alike in the eyes of God: everything from a white lie or a child stealing a cookie to mass murder. They believe this not out of common sense, but because they erroneously think that the Bible teaches it. This mistaken notion is decisively refuted by the following biblical passage:
1 John 5:16-17 (RSV): "If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. All wrongdoing is sin, but there is a sin which is not mortal."People are not always completely aware that certain acts or thoughts are sinful. In Catholic theology, in order to commit a grave, or mortal sin, where one ceases to be in a state of grace and is literally in potential, but real danger of hellfire, three requirements are necessary: 1) it must be a very serious matter, 2) the sinner has to have sufficiently reflected on, or had adequate knowledge of the sin, and 3) he must have fully consented in his will. Scripture provides many indications of this difference in seriousness of sin, and in subjective guiltiness for it:
Luke 12:47-48: "And that servant who knew his master's will, but did not make ready or act according to his will, shall receive a severe beating. But he who did not know, and did what deserved a beating, shall receive a light beating. Every one to whom much is given, of him will much be required; and of him to whom men commit much they will demand the more." (cf. Lev. 5:17, Lk. 23:34)The Bible also refers to (mortal) sins which - if not repented of - will exclude one from heaven (1 Cor. 6:9-10; Gal. 1:8; Eph. 5:5; Heb. 12:16; Rev. 22:15).
John 19:11: "'. . . he who delivered me to you has the greater sin.'"
Acts 17:30: "The times of ignorance God overlooked, but now he commands all men everywhere to repent," (cf. Rom. 3:25)
1 Timothy 1:13: "though I formerly blasphemed and persecuted and insulted him; but I received mercy because I had acted ignorantly in unbelief."
Hebrews 10:26: "For if we sin deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins,"
But what about James 2:10?: "For whoever keeps the whole law but fails in one point has become guilty of all of it." Doesn't that prove that all sins are the same; equally destructive and worthy of judgment?
This passage deals with man's inability to keep the entire Law of God: a common theme in Scripture. James accepts differences in degrees of sin and righteousness elsewhere in the same letter: "we who teach shall be judged with a greater strictness" (3:1). In 1:12, the man who endures trial will receive a "crown of life." James also teaches that the "prayer of a righteous man has great power in its effects" (5:16), which implies that there are relatively more righteous people, whom God honors more, by making their prayers more effective (he used the prophet Elijah as an example). If there is a lesser and greater righteousness, then there are lesser and greater sins also, because to be less righteous is to be more sinful, and vice versa.
This distinction in the character of sins, viz. that some argue absence of faith and involve the loss of God's favour, and that others do not, is a very important one to insist upon, even though we cannot in all cases draw the line and say what sins imply the want of faith, and what do not; because, if we know that there are sins which do throw us out of grace, though we do not know which they are, this knowledge, limited as it is, will, through God's mercy, put us on our guard against acts of sin of any kind; both from the dread we shall feel lest these in particular, whatever they are, may be of that fearful nature, and next, from knowing that at least they tend that way. The common mode of reasoning adopted by the religion of the day is this: some sins are compatible with true faith, viz. sins of infirmity; therefore, wilful transgression, or what the text calls "departing" from God, is compatible with it also. Men do not, and say they cannot, draw the line; and thus, from putting up with small sins, they go on to a sufferance of greater sins. Well, I would take the reverse way, and begin at the other end. I would force upon men's notice that there are sins which do forfeit grace; and then if, as is objected, that we cannot draw the line between one kind of sin and another, this very circumstance will make us shrink not only from transgressions, but also from infirmities. From hatred and abhorrence of large sins, we shall, please God, go on to hate and abhor the small.