Yes, the "wall of separation" between church and state is a matter of interpretation, but it seems to me it was the interpretation at least some of the writers had in mind. TJ, f'rinstance, wrote thus to the Baptist Congregation of Danbury:
<blockquote><i>Believing that religion is a matter which lies solely between man and his God, that he owes account to none other for his faith or his worship, that the legislative powers of government reach actions only, and not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their Legislature should "make no law respecting an establishment of religion, or prohibiting the free exercise thereof," thus building a wall of separation between Church and State.</i></blockquote>
So Jefferson both repected a God, and wanted to keep him out of the actual practice of the gov't. I see no contradiction there. And that's where "Wall of Separation" comes from. Not the constitution, not legal proceedings. From one of the authors explaining it afterwards. Doesn't come much clearer than that.