Truth

How did the Church of England decide that contraception was not sinful?
How did the Church of Scotland decide that openly-gay and non-celibate gay men and women could be "ordained" as ministers?
How the the Anglican Church in Canada decide to bless and accept gay unions?
How did practically any Protestant church decide what is truth or not? A vote.

How does that Catholic Church decide truth? It's slowly revealed to us by God.