Unchanging doctrine?

Round about the year 50CE the church had to get together to resolve an serious argument among its leaders about what counted as it's teaching and what didn't. In the middle ages the pope changed the definition of the terrible sin of usury when he realised that they needed to benefit from capital investments. In 1633 the Roman Catholic church condemned Galileo for teaching that the Earth and the planets went round the Sun, which it finally recanted in 1992. For more than 1800 years the church did not blink at slavery, which is accepted throughout the bible, and which it now condemns. In 1930 the Church of England reversed it's position on contraception. When I was a clergyman I was not allowed to remarry divorced people. Since 2002 that changed (grudgingly).

So what is anyone doing saying that the church's doctrines come from God and cannot be changed? Even the central teachings about Jesus himself come from an argument between St Paul and James, Jesus' brother, which Paul happened to win. How can Mr Welby pretend to be holding the line on homosexuality when he must surely realise that history is against him? The same Church of England fought itself to a standstill in my time as a clergyman over the ordination of women, which it now accepts (although still resisted in some quarters), and is doing it again over women bishops.

Is there something that prevents the religious looking at their own history and learning from it?