John Calvin Commentary 1 John 5:10

"He that believeth on the Son of God hath the witness in him: he that believeth not God hath made him a liar; because he hath not believed in the witness that God hath borne concerning his Son." — 1 John 5:10 (ASV)

He that believeth not—as the faithful possess this benefit, that they know themselves to be beyond the danger of erring because they have God as their foundation, so He makes the ungodly guilty of extreme blasphemy because they charge God with falsehood. Doubtless, nothing is more valued by God than His own truth; therefore, no wrong more atrocious can be done to Him than to rob Him of this honor.

Then, to induce us to believe, He takes an argument from the opposite side. For if making God a liar is a horrible and execrable impiety, because then what especially belongs to Him is taken away, who would not dread withholding faith from the gospel, in which God would have Himself be counted singularly true and faithful? This ought to be carefully observed.

Some wonder why God commends faith so much, and why unbelief is so severely condemned. But the glory of God is implicated in this; for since He designed to show a special instance of His truth in the gospel, all those who reject Christ offered to them there leave nothing to Him.

Therefore, though we may grant that a man in other parts of his life is like an angel, yet his sanctity is diabolical as long as he rejects Christ. Thus, we see some under the Papacy vastly pleased with the mere mask of sanctity, while they still most obstinately resist the gospel.

Let us then understand that it is the beginning of true religion to obediently embrace this doctrine, which He has so strongly confirmed by His testimony.