Mark 16:16
He who shall believe and be baptized shall be saved. This promise was added in order to allure all mankind to believe; as it is followed, on the other hand, by a threatening of awful destruction, in order to terrify unbelievers. Nor is it wonderful that salvation is promised to believers; for, by believing in the only begotten Son of God, not only are they reckoned among the children of God, but receiving the gift of free justification and of the Spirit of regeneration, they possess what constitutes eternal life. Baptism is joined to the faith of the gospel, in order to inform us that the Mark of our salvation is engraved on it; for had it not served to testify the grace of God, it would have been improper in Christ to have said, that they who shall believe and be baptized shall be saved. Yet, at the same time, we must hold that it is not required as absolutely necessary to salvation, so that all who have not obtained it must perish; for it is not added to faith, as if it were the half of the cause of our salvation, but as a testimony. I readily acknowledge that men are laid under the necessity of not despising the sign of the grace of God; but though God uses such aids in accommodation to the weakness of men, I deny that his grace is limited to them. In this way we will say that it is not necessary in itself, but only with respect to our obedience.
But he who shall not believe shall be condemned. By this second clause in which Christ condemns those who shall not believe, he means that rebels, when they reject the salvation offered to them, draw down upon themselves severer punishment, and not only are involved in the general destruction of mankind, but bear the guilt of their own ingratitude.