Subjection to the royal authority of Christ–the spiritual kingdom–is possible under whatever circumstances one finds oneself according to the flesh; for “the Kingdom of God is within you.”
Thus the claim of the chief priests in John 19 that “we have no king but Caesar” is erroneous and dangerous more than simply because it means the rejection of salvation. It also indicates a fundamental misunderstanding of the Kingdom of God and how it relates to the kingdoms of this world. The claim that “we have no king but Caesar” shows that the chief priests believe (or say that they believe) that the kingdom is an external matter: that one cannot be subject to Christ and to the government.
For Calvin, however, that is not the case. The Kingdom is “spiritual” (it does not consist in external forms, circumstances, or orders), and turning away from Christ as King–the one who (alone) wields “royal authority” in this Kingdom–means turning away from that spiritual kingdom and from willing submission to Christ’s just rule. The one who does so loses both the salvation and the happiness he could have under any conceivable external form of earthly government.
This does not prevent one from distinguishing between the rectitude of various types of government: Calvin is very clear that some governments are “just and lawful” while others are tyrannical (and the Romans, he says, were tyrants).
It means only that the Kingdom is not any of these earthly and external things; it is “not a matter of eating and drinking.” Hence we can have the joy of the Spirit under the kingly rule of Christ, whatever our circumstances may be.
15. We have no king but Caesar. This is a display of shocking madness, that the priests, who ought to have been well acquainted with the Law, reject Christ, in whom the salvation of the people was wholly contained, on whom all the promises depended, and on whom the whole of their religion was founded; and, indeed, by rejecting Christ, they deprive themselves of the grace of God and of every blessing. We see, then, what insanity had seized them. Let us suppose that Jesus Christ was not the Christ; 165 still they have no excuse for acknowledging no other king but Caesar. For, first, they revolt from the spiritual kingdom of God; and, secondly, they prefer the tyranny of the Roman Empire, which they greatly abhorred, to a just government, such as God had promised to them. Thus wicked men, in order to fly from Christ, not only deprive themselves of eternal life, but draw down on their heads every kind of miseries. On the other hand, the sole happiness of the godly is, to be subject to the royal authority of Christ, whether, according to the flesh, they are placed under a just and lawful government, [or whether they are] under the oppression of tyrants.