Since the orginial founders of scientific socialism (Marx and Engels) were clearly opposed to religion and nothing has happened since which proves that their criticisms were wrong, why are some socialist groups so supportive of religion? Are they just trying to attract more members? If so, why don't they just say that they've given up the whole idea of a socialist revolution and that they instead want to make minor reforms to the capitalist system? That would win these groups a lot more supporters? So why is it that there is so much support for religion among certain socialist groups? Any ideas?
You nailed it. Opportunism at worst. Non-materialist ignorance at best.
As far as the US goes, a lot of groups developed with a Christian foundation. First, they felt that since a lot of workers were religious they would have to incorporate a sense of "spiritual morality". Another factor, again in the US, was that many of the labor, progressive, socialist leaders in the 1860's-1890's didn't really understand a scientific approach. Many leaders tried to continue early American ideals which meant preserving the present traditions and outlook, rather than offering a new approach/lens. Eugene Debs used a lot of Christian rhetoric and moralism in his speeches and pamphlets. I can't speak for other countries, not my area of knowledge.
