What other God is there?

Christianity came into existence in a polytheistic world, where belief
in the existence of many gods was commonplace. Part of the task of early
Christian writers appears to have been to distinguish the Christian God from
other gods in the religious marketplace. 

At some point, it had to be asked which god Christians were talking
about, and how this god related to the “God of Abraham, Isaac, and Jacob,” who
figures so prominently in the Old Testament. 

The doctrine of the trinity appears to have been, in part, a response
to pressure to identify the god that Christian theologians were speaking about.

As time passed, polytheism began to be regarded as outdated and rather
primitive, especially within the sophisticated intellectual culture of the
cosmopolitan city of Alexandria.

The assumption that there was only one god, and that this god was
identical to the Christian God, became so widespread that, by the early middle
ages Europe, it seems self-evident.

Thus Thomas Aquinas, in developing arguments for the existence of God
in the 13th century, did not think it worth demonstrating that the God whose
existent he had proven was the “God of the Christians,” after all, what other
God was there?