Cult of the Lamb

Can confirm, achievement requires upgrading the temple fully.

I did upgrade fully, and mine didn’t unlock.

That's wrong, you're talking about Step IX, Step X is last one and costs 15 sin and 50 gold bricks

Naked party and drinks works well but there's also confession option which give some sin boost.

However best option is to raise sin from all followers just before filling the bar the avoid overloading them with sin, getting new member from crusade or just buy him and then filling him with sins of rest of cult. That gives max 3 sin points after the ritual and 1 more sin point after you deal with him during crusade

Also worth mentioning during crusade you can get free sin point after beating bosses, requires the right doctrine and is not a guarantee