Is a 6:30 wedding not a church likely formal or informal?

Yes it is likely to be one of the other. It is "YOUR" wedding so you make it what you want and then inform the quests

Sounds like you're a guest at this wedding and are trying to figure out what to wear? After 5 is typically more formal than before 5. Whether it's at a church doesn't really impact how formal it is. I would assume it's somewhat formal - something like a dressier (not business) suit or cocktail dress for a woman, a suit for a man (or slacks, tie, sportcoat if you don't have a matched suit). You can't really go wrong.

I've rarely been invited to weddings that people did not dress up for. If they don't specifically say casual on the invitation, I would assume you should dress up for it. Even if they do say "casual", that still means something nicer than your usual jeans for a wedding usually - like golf attire for men, daytime dresses/skirts or slacks (cruise wear kind of stuff) for women.

Have a good time.

The time alone can not tell us how formal or informal the wedding is. What kind of invitation did you receive? Where is the reception being held? Are you familiar with this venue? If not, call them and ask if they have a dress code.

You must know the bride and/or groom pretty well. Are they more likely to have a wedding that is formal or informal?