There are those who, reading Guénon and Evola, or even Dugin, take a Perennialist view of true religion. Those of the Traditionalist School especially see the world’s traditional religious traditions as sharing a single, metaphysical truth, from which they suppose that all esoteric and exoteric knowledge and doctrine have grown. For this reason, they also sometimes oppose Christian evangelism, believing that such endeavours endanger the peace of the world for the sake of sectarian ends. I’m sympathetic to Perennialism, but I don’t consider myself a Perennialist.
For one thing, though I do think human knowledge of God has come down from Noah in (almost) every society and been built upon through experience in various societies, there is no knowledge of God like the revelation of God’s only-begotten Son himself, come to us in the flesh, crucified for us, raised from the dead according to the Scriptures, now reigning as God-king from heaven. It is for this knowledge, and right worship grounded on it, that the Christian is called to make all nations into disciples of Christ: not lest every last man who has not heard of the work of Christ should suffer everlasting perdition, but so that those who hear the gospel of Jesus Christ and reject it may be judged by everlasting justice, while those who hear it and believe may be sanctified and made just in this life, to the glory of God the Father.
And though it is certain that many have been saved through an implicit faith in Christ who have never heard of him, a Perennialist doctrine that would deny the full truth of the Scriptures’ exoteric message, namely the gospel, would be a road not to greater spiritual depth but to the depths of hell. For the Bible is full of esoteric things, at which even the most erudite can only point and conjecture, with greater or lesser probability; but greater than all of these esoterica is the gospel, simple enough for a child and deep beyond the most learned seraph. The Lord said against the Jews of his generation, whom he called evil and adulterous, The queen of the south shall rise up in the judgment with the men of this generation, and condemn them: for she came from the utmost parts of the earth to hear the wisdom of Solomon; and, behold, a greater than Solomon is here. The men of Nineve shall rise up in the judgment with this generation, and shall condemn it: for they repented at the preaching of Jonas; and, behold, a greater than Jonas is here. It will be no better for those who know what the gospel of Jesus Christ says, but for their own sophistication will not confess Christ truly, but go to church only because their fathers did, believing that God is not truly and literally come in the flesh to save sinners and join them to himself. Their own fathers will rise up in the judgement and condemn their folly. But if they believe and do not deny the Lord in their hearts, they will be his people, and he will be their God.