Which one? The pre-VCII Magisterium or the post-VCII Magisterium?
If one has to ask, then one have a series of problems and questions.
In short, if one don’t believe John XXIII was a valid Pope, calling a valid Council, that would be at least, problem #1.
If one doesn’t believe in the teachings of the Council, as approved by Paul VI, that is another problem.
If one is a cafeteria Catholic, picking and choosing what they believe and don’t believe, whether that artificial contraception is OK, or failure to wear a chapel veil is not ok, the problem is all the same.
1842 By faith, we believe in God and believe all that he has revealed to us and that Holy Church proposes for our belief.
One has to ask, do I believe in what Holy Church proposes for our belief, or not. That is the crux.