Using 1925 kind of throws me off. I have little booklet on Presbyterian history and I find many divisions of the church. I found north in two divided parts that merged together in the North and I found two south in two parts that merged together and I found north and south merged together and merging other reformed Presbyterian churches after modification the old confessional creeds to satisfy both parties. The popular PCA split off this and now hearing rumors among PCA on splitting again. And at the same time the Presbyterians in the England, France, Germany area are very different than in America.
What do you mean by "stable" since 1925?
Up to the early 1920s, the predecessor denomination attempted to enforce the 5 "fundamentals", the virgin birth, blood atonement, etc. (The term is misleading because it's really the fundamentalist distinctives, not the key doctrines such as the Trinity and Incarnation).
Around 1925 (actually 1924, I believe), a number of theologians from both conservative and liberal perspectives published the "Auburn Affirmation," saying that although some of them accepted those ideas, none of them considered the ideas essential. One of those ideas was the inerrancy of Scripture.
Very quickly, the Church effectively accepted that. It doesn't mean that everyone abandoned inerrancy, but that it was fairly widely agreed that it wasn't essential. In reaction to this and associated events, the most conservative members formed the Orthodox Presbyterian Church, in 1936.
One key event in this was the rejection of J. Gresham Machen, who was a well-known conservative. He started his protect around 1929, and was ejected in 1935. (I believe the way he was treated was a big mistake, although I also disagree with his theology.)
Obviously the changes took some time to play out, but I'd say that the Auburn Affirmation in 1924 is the best place to draw the line. After that, the PCUSA at least accepted theological approaches that aren't very far from today's theology, although the issue of gays hadn't come up then.
I wouldn't want to say that there have been no new ideas since 1925, but I would say that since that time the church has fairly consistently accepted moderately liberal ideas. The period of time when G-6.0106b was in effect was, in my opinion, an exception, during which a literalist approach to that one issue was mandated. (Actually, you could consider starting that clock at 1989.) But that approach really only affected the approach to homosexuality, not the overall theological tenor of the denomination.
You're right that there were various mergers. Some of the groups were slightly more conservative before merging. But I think the northern church has maintained a fairly consistent position of permitting positions that do not accept inerrancy or the ideas that flow from it, while maintaining a basically Nicene position (although with a more flexible view of what the Trinity means than in some more conservative churches).
