If Pope St. Pius X was correct that “modernism is the synthesis of all heresies” then wouldn’t we see all heresies of the past fulfilled in it?
Yes we would see that. And yes we do see that
If Jesus is merely the “privileged route to salvation,” then all religions can be a vehicle to salvation. However, this notion is no different from Arianism. Here’s why: Jesus claims to be God. Jesus claims to be the only way, truth and life. If this is not true, then He is not God. And this is Arianism. Thus, religious indifferentism encapsulates a thousand heresies, including Arianism.
Notice that belief in the Trinity is inherently linked to the Catholic Faith being necessary for salvation (with implicit desire for baptism occasionally being enough for salvation.) But according to St. Athanasius, the Catholic faith is the One Faith which everyone must keep “whole and undefiled [or] without doubt he shall perish everlastingly.”
The Creed of St. Athanasius begins thus:
Whosoever will be saved, before all things it is necessary that he hold the Catholic Faith. Which Faith except everyone do keep whole and undefiled, without doubt he shall perish everlastingly. And the Catholic Faith is this, that we worship one God in Trinity and Trinity in Unity. Neither confounding the Persons, nor dividing the Substance. For there is one Person of the Father, another of the Son, and another of the Holy Ghost. But the Godhead of the Father, of the Son and of the Holy Ghost is all One, the Glory Equal, the Majesty Co-Eternal. Such as the Father is, such is the Son, and such is the Holy Ghost.
St. Athanasius wrote this against Arius. Notice therefore that the notion that Christ is merely the “privileged route to salvation,” is not only a denial of the necessity of the Catholic faith for salvation, but also a denial of the Trinity. Why? Again, as I wrote above, Jesus claims to be God. Jesus claims to be the only way, truth and life. If this is not true, then He is not God. And this is Arianism. Thus, modernist religious indifferentism encapsulates a thousand heresies, including Arianism. This means that Pope St. Pius X is correct: “Modernism is the synthesis of all heresies.” I add: This includes Arianism, as religious indifferentism is truly tantamount to a denial of the Trinity as the one true God if other religions can get you to heaven.
Or, if this seems a stretch, just read again the first sentence in bold from a canonized doctor of the Church fighting against those who deny the Trinity: Whosoever will be saved, before all things it is necessary that he hold the Catholic Faith.