You’re flat out wrong when it comes to the Roman Catholic Church - I don’t know enough about Islam to say whether you’re right about that.
In church doctrine, Matthew 16:18 and 16:19, and again in Matthew 18:18, give ultimate authority to St Peter (the first Pope) and all the Popes that followed him.
Essentially the Pope can decide whatever, and it just is. Tomorrow the Pope could decide that gay marriage and abortion are a-okay, and they would be a-okay as far as heaven is concerned.
He might get lynched and the next Pope reverses it, but that mechanism for change exists, and has been used many times in the past - one notable recent one was when the Pope decided dogs go to heaven, so now dogs go to heaven.
Source: ex-Christian who was very involved within the Church institution.