I have not been to a Mason meeting but my dad was high up in the lodge. Ultimately I talked him into realizing the nonsense that it was and he accepted Christ as his Savior and became a Christian. I believe you are incorrect when you imply that Masons believe in one God. I believe what my father told me was that in order to belong you had to believe in a Supreme Being meaning you could also believe that that Supreme being was Allah which is garbage in should not be condoned anywhere. And any Lodge or any institution that contributes to such misinformation should be condemned in my opinion.
Perhaps your Dad wasn’t a Christian. I don’t really care. But the Masons aren’t a religion, it’s a civic organization. If your father was “high up” in it and didn’t understand that...then he must not have been paying much attention.
His acceptance of Christ wouldn’t exclude him from being a mason. And if your church opposed it, then that doesn’t make a lot of sense.