True and authentic christianity is between the believer and God. You can go to any church, Catholic or not, and not be a Christian. God knows His own and His sheep hear His voice. Church is desirable for sure, but it doesn’t make you a Christian just by being part of it.
Justification is a process that is between the believer and God. True Christianity is Catholic Christianity because it is conformed to the Sacred Deposit of Faith Christ gave the Apostles in order to teach the world.
It is, of course, true that many Catholics lose their salvation despite the Church’s efforts, but it is foolish and prideful to turn away from the Church purposely; hence this Open Letter.
Very well put!