Pope Pius X described 'Modernism' in Encyclical Pascendi Dominici Gregis: “And now, can anybody who takes a survey of the whole system be surprised that We should define it as the synthesis of all heresies?”

What did he mean by this?

Well, the word 'synthesis' means 'the combining of separate elements to form a coherent whole'. However, Pius X was not saying that Modernism was a profession of every single historical heresy, something mentally impossible, since many of them are mutually exclusive. Rather, he was describing a system that "lay the ax" to the very root of the faith like each of these historical heresies did.

So what is this 'system'?

This system is an alliance between faith and false (worldly) philosophy; an alliance between Christianity and Gnosticism; an alliance between the Tree of Life and ...

The Protestant revolution was a consequence of the Renaissance which was a consequence of the Eastern Schism which was a consequence of pseudo Christian thinkers infiltrating the Byzantine Church. It just goes back, and back and back. The same old system: rational thought of fallible men dictating to the faith. Really, I think Protestantism has played the greatest role in history - it really is a culmination of many of the earlier heresies. Modernism is its parent but in a far more subtle form. A good book to read on it is Michael Davies' 'Partisans of Error'. It s nice, easy read - only around 100 pages - and gives a quick summary of the influences involved. The following link also gives a simple overview of the historical heresies: https://www.veritascatholica.com/the-great-heresies-of-the-church-2/

Can you post me a link to your post about 'Principals of Liberalism' or is it too far back?

So, if you examine the different facets of Modernism, they have one aspect in common ... making the "rule of faith" an internal one, based on one's personal experience of faith, where truth is not objective, fixed, immutable ... but rather relative to the observer, the believer, and his own faith experience.  Similary, the Church's understanding or experience of the faith can change over time.

It's the synthesis of all heresies in that, at the end of the day, the elimination of the one, true, objective rule of faith ... and its replacement by another, which, as St. Thomas writes, ultimately ends up being one's own mind.  Prots say the Bible is their rule of faith, but it reduces in the end of their own interpretation of the Bible.  Prots said that the Holy Spirit will inspire the believer with the true interpretation or meaning of the Bible, and now there are 27,000+ such true interpreations.

In all heresies, man's own mind becomes his own proximate rule of faith, even if some of the earlier heresies claimed that there was a rule of faith external to them, e.g. the Bible, the Ecuмenical Councils (but not Popes), etc. ... but these reduced to their own understanding of these rules, and this culminates in Modernism, which holds that the rule of faith is in fact internal to man.

Man becomes his own rule of faith, man his own rule of moral conduct, man the rule of his own reality ... man becomes his own god.  That is the ultimate spirit of Antichrist.