Should've been clandestine Satanists not renegade Catholics
Part of the Antichrist's allure is that he's born with evil intent by the "parent" but as they made him now, given that they manage to control him, he would be doing the churches bidding (as they see it). It makes no sense for Damien to be commissioned by people trying to improve their Christian congregation.
A better way it would fit is that they were clandestine Satanists trying to subvert the church instead.