What about using right reason only, and dispensing with scriptural proofs for Catholicism.
With all due respect to the Almighty, let me just ask:
If God is sane and wishes his message to be spread, why would he want two, three or two hundred thousand competing Christian churches?
Logic dictates that He would want one to do so.
Logic dictates that if we exist here, with obvious free will and ample choice, then this sane creator has allowed us to sin and separate from each other.
Why would a logical God, and a compassionate one, want harm to befall his most advanced, most wondrous creatures?
And so why would he want us to choose schism - which must logically be a sin - the willful separation from the body of believers?
Protestantism has enough holes in it, but if you believe that there is a God, why would he want more than one way of worshipping Him and living on this earth?
No, the Church is One, Holy, Catholic and Apostolic. It has apologized for its role in what caused these schisms. But that does not change the fact that schism is intrinsically wrong.
There is only one Church as there is only one Lord of it. It is not only a mystical church, but a hierarchical one as well - unless you believe that Jesus was preparing all his apostles and disciples to be organized and in communion for their lifetimes only.
I know who my authority is.