Gradual erosion.

When so called Christian Socialists met together at the end of the 19th Century with other socialists such as The Fabian Society, there was apparent common ground but the large numbers of atheists ensured that a marxism-inspired 'destruction of religion' - and particularly of Christianity - was high on the hidden agendas of many.
There was a seeming harmony and unity of purpose but it was always the intention of the unbelievers to use this movement for the obliteration of faith.
It took very little time before 'social darwinism' was created as a means to reduce the importance of Christianity in the movement.
Christians in early socialism were forced to begin compromising and this erosion of true faith within socialism has carried on over generations with that old philosophy of 'softlee, softlee catchee monkey' as its methodology.
If being a Christian in leftwing parties of the past was just about possible - we have now reached a time of burgeoning leftwing persecution of the Church and its adherents which no longer permits this.
To be a supporter of any one of the leftie parties today - you need to make a choice. It really is an 'either or'!

You have to realise that the left is no longer interested in justice - if indeed they ever were. They have had to redefine it!
Remember - 'liberal christianity' is no Christianity at all.
Clever tactics undermine faith by suggestions of sidelining Jesus and attempting to enter heaven solely with 'good works'.
Sorry - that don't work!