I am not one who believes that the only sin is rejecting Christ. Rejecting Christ as the only way of salvation is foolish and fatal. But when you stand before God without Christ as your substitute you will be punished for your own sins.
One does not alter the other.
Judas was already a thief, stealing money and even trying to get more money ostensibly to give to the poor, (Jn. 12:6) but his ultimate sin was that of willfully betraying Christ, even after being warned it would have been better off not to have been born if he did so. (Mk. 14:21) In so doing he evidenced that he was of the devil, as he "put into the heart of Judas Iscariot, Simon's son, to betray him," and thus was a devil. (Jn. 6:70; 13:2)
While this, as well as all things, was foreordained of God who knows all things, it does not remove the persons free will, acting according to their sinful nature, and in rejecting light and grace given them to varying degrees that they should not.
According to which they shall be judged, and as Judas was personally discipled by Christ, so he is highly accountable, and like cites which saw more grace than Sodom, (Lk. 10:12) and the Scribes and Pharisees, he shall receive the "greater damnation." (Mk. 12:40) Which also means Christians are more accountable than the lost. (cf. Lk. 12:48)
Now if popes engaged in such Scripturally substantiated teaching than it would find evangelical commendation, but we do not expect that since doctrinally Scripture is more of a mere servant to wrest support from for traditions.
That sin is by default rejecting Jesus is obviously a Catholic teaching because it certainly isn't found in Scripture.
We are told that we all sin, all the time and that if we claim we are without sin, we deceive ourselves.
If sinning = rejecting Jesus, then nobody COULD be saved.
Fortunately, that is not the case. We are not saved, lost, saved, lost....... in an on again, off again works based salvation.
When we are saved, we are saved, sealed by the Holy Spirit, transferred into the kingdom of the Son He loves.
So we still sin anyway? Sure. But it doesn't cost us our salvation that we have to earn back every time we slip up or even deliberately sin in a moment of weakness.
No parent disowns their child every time the child disobeys him or her. The child is still the child and loved as such, even in the midst of disobedience and rebellion.
It breaks the parent's heart, but no parent who loves their child disowns them for it.
Catholics need to learn about the love of a father for his children and the love of THE Father for HIS children because they just don't get it. The Catholic God is a harsh taskmaster.