If Catholics would break out of the denominational mindset that they are in and really understand that the body of Christ is NOT a denomination, but rather the the individual true believers of Him irrespective of church affiliation, they would find it less incredulous.
However, being conditioned to think that a man made denomination is the “true” church, it is impossible for them to see it. Catholics need to learn to think out of the box they’ve been out in and that takes a work of the Holy Spirit.
So, ask Him to give you understanding of it. He will.
Yeah OK, thanks for your concern.
Consider denominations like branches of a family tree. In this case, the Christian family tree.