If we accept the doctrine of the Fall, then we know that all men's eyes are blind, and ears deaf to the truth of God - and perceptions of God's works in nature are corrupted. Therefore we should readily stand with Calvin when he proclaims that all religion that is grounded on the revelation of God in nature, and on men's reasoning therefrom, is necessarily false. He writes:
Paul declares that the Ephesians were without God until they learned from the gospel what it was to worship the true God [Eph. 2:12-13]. And this must not be restricted to one people, since elsewhere he states generally that all mortals "became vain in their reasonings" [Rom. 1:21] after the majesty of the Creator had been disclosed to them in the fashioning of the universe. For this reason, Scripture, to make place for the true and only God, condemned as falsehood and lying whatever of divinity had formerly been celebrated among the heathen; nor did any divine presence remain except on Mt. Zion, where the proper knowledge of God continued to flourish [Hab. 2:18, 20]. Certainly among the pagans in Christ's lifetime the Samaritans seemed to come closest to true piety; yet we hear from Christ's mouth that they knew not what they worshiped [John 4:22]. From this it follows that they were deluded by vain error. (p. 67, Institutes of the Christian Religion)Religions abound today that are "peaceful" and "kind" to others... but even that faith that is dedicated to serving others is sin - apart from the revelation of Jesus Christ. There can be much good ethical behavior in the world, among those even who claim no god - but again it is not to be approved of, where the true and living God is not praised and submitted to as King.
It is therefore no wonder that the Holy Spirit rejects as base all cults contrived through the will of men; for in the heavenly mysteries, opinion humanly conceived, even if it does not always give birth to a great heap of errors, is nevertheless the mother of error. And though nothing more harmful may result, yet to worship an unknown god [cf. Acts 17:23] by chance is no light fault. Nevertheless, by Christ's own statement all who have not been taught from the law what god they ought to worship are guilty in this matter [John 4:22]. (p. 67, Institutes of the Christian Religion)Calvin continues in the last two sections to argue that there is no excuse for those who have followed their own whims and created their own gods, nice though they might be. God has revealed his Holy attributes to men (Romans 1:19) and men, through their fallenness and consequent inability to see clearly that revelation, have not sought out the true and living God to worship Him aright. The fault lies not in God, nor in His common revelation in creation, but upon men themselves.
Calvin doesn't shy away from calling those who do not worship the God of Scripture as professors of a false and condemned religion. He doesn't make excuses, where there are none, for those who have not professed faith in Jesus Christ - rather, he, like Paul before him, states plainly that he who does not worship the Lord God in spirit and truth according to His Word, is standing before God unprotected from the just wrath which falls upon him for his sin. Calvin doesn't try to construct some sort of 'fairness doctrine' or plea on behalf of the one who doesn't know God, but is trying to serve humanity by being kind to everyone. I'm quite certain Calvin would be looked upon as a bigot and by some even a blasphemer for suggesting that not every religion is a true path to salvation. I'll stand with Calvin, though and together with him on the pure Word of God whose witness on this matter is clear as a bell, and thank Him for having mercy upon us so that we are led to know Jesus Christ His Son and our salvation, for we never would have aimed for or found Him on our own.