The most horrifying thing is that after you pick the optimal faith, you need to unbelieve everything you considered during your choice while keeping the belief that you have chosen right and acquiring the belief in the chosen faith.
The reason is that when choosing you suppose that everyone actually worships the same God, and you need to find just an optimal way of worship; but the closest faiths to it—I think they are Bahaism and most factions of Buddhism—are easy to satisfy by default, you need to believe not them, but one of the probably-least-true faiths—one of strict jealous faiths. Simply because the faith you considered right while choosing is permissive enough to forgive your move into a strict one you will choose to believe a faith which is furthest from your initial assumption.
Separate comment for a separate point.
The most horrifying thing is that after you pick the optimal faith, you need to unbelieve everything you considered during your choice while keeping the belief that you have chosen right and acquiring the belief in the chosen faith.
The reason is that when choosing you suppose that everyone actually worships the same God, and you need to find just an optimal way of worship; but the closest faiths to it—I think they are Bahaism and most factions of Buddhism—are easy to satisfy by default, you need to believe not them, but one of the probably-least-true faiths—one of strict jealous faiths. Simply because the faith you considered right while choosing is permissive enough to forgive your move into a strict one you will choose to believe a faith which is furthest from your initial assumption.