You start with 10 points when you sign up.
Every "thumb up" you get 1 point.
Every "thumb down" you lose 1/3 of a point.
The system only displays whole numbers; the "fraction" is stored in your account but no one sees it.
And, of course, your critics are only allowed to be 16% or 17% of your total downvotes. Once they hit that limit, they'll get a message saying why they can't thumb you down any more.
When you find yourself an enemy on here, you're certain to hit that limit :) But, on the other hand, when you're trucking along being a good Catholic and YOU manage to make an enemy, you'll be glad your "foe" can't single-handedly destroy your rep score.
The system is rigged heavily in favor of participating on the forum. If you participate a lot, you're almost guaranteed to increase your rep score. Unless your post is grossly unpopular (as in, more than 3 times as many people hate the post than like it) you will come out ahead. You could get 8 downvotes and only 3 upvotes, and still come out ahead.