The argument of Pope Liberius is often used against us. It cannot be compared. Here is what St. Robert Bellarmine said of it:
"Then two years later came the lapse of Liberius, of which we have spoken above. Then indeed the Roman clergy, stripping Liberius of his pontifical dignity, went over to Felix, whom they knew [then] to be a Catholic. From that time, Felix began to be the true Pontiff. For although Liberius was not a heretic, nevertheless he was considered one, on account of the peace he made with the Arians, and by that presumption the pontificate could rightly [merito] be taken from him: for men are not bound, or able to read hearts; but when they see that someone is a heretic by his external works, they judge him to be a heretic pure and simple [simpliciter], and condemn him as a heretic."
This Felix is venerated by the Roman Church as Pope and Martyr.
Liberius did not preach heresy, but sinned by making peace to the arians to safeguard his life. Even then the Roman clergy was justified, according to Bellarmine, in taking the pontifical dignity from Liberius and giving it to Felix, because of the impression of heresy Liberius gave.
Otherwise the Church has been wrong for more than thousand years in venerating an antipope as Pope and Martyr.