While Gen. Rab. 11.6 asserts that Adam was never circumcised, the Midr. Ps. 9.7 says he was born circumcised. ‘Abot de-Rabbi Nathan (A, 2) says the same, but adds that the reason for it is that Adam was created in the image of God (citing Gen 1:27). According to the Mishnah, a body without circumcision was imperfect (Nedarim 3.11):
Rabbi says: Great is circumcision, for despite all the religious duties which Abraham our father fulfilled, he was not called ‘perfect’ until he was circumcised, as it is written, Walk before me and be perfect.
It seems some Rabbis thought of God as circumcised. Interesting.