Hobbies. I struggled with this after college also because I left the church and my old social life had dried up. I was conditioned to just show up and my friends would be there and the nonreligious outside world doesn't always work that way.
I would join a club or a group, preferably one with an even gender split or even a skew towards whoever you're interested in dating. I found dancing in 2006. I never would have expected to get into it and probably spent most of the 90s calling it "gay". I was tricked into going by a friend who said we were going bowling. I trusted her and she drove, so I had no escape. Many dances are "social" dances which means anyone can ask anyone to dance and you aren't expected to bring a partner, most people don't. I kept doing it and eventually started going without my friends. 20 years later, I have been in charge of running dances, I've been on the committee of large events, I've made some money teaching lessons, but most importantly of all, I've collected a circle of awesome supportive people, some acquaintances, some friends, and a handful that I've dated. Don't go in with the intention of dating though because it counter intuitively guarantees you won't find a date. Instead, just have fun. Ask the people who aren't getting asked to dance, make friends, enjoy the music, etc. People notice when someone is capable of having fun on their own and they want to be a part of that. They appreciate someone who will dance with the sweet little old lady who shows up every week and not just the 10/10 blonde with the double D's. My goal was always to dance with every woman in the room once, then go back for seconds with the people I most enjoyed dancing with. It can cost a few bucks to get in, but almost all of them will let you in free if you volunteer for a half hour to collect admissions or help set up/ tear down. It's harder for guys (if you dance the lead role) to get started, but don't be discouraged because we're outnumbered and always in demand.
Biking is another good activity to meet people. You can join a club in many cities for a few bucks and they'll basically send upcoming rides to your inbox all year round. If you're not exercise inclined, there are also PEV (personal electric vehicle) rides in many cities that give you all of the fun and exploration with only a fraction of the workout.
Other good ideas: Frisbee golf league, ultimate Frisbee, hot springing (hot spring hippies are cool and very welcoming), poetry slams, board game parlours (these seem to be popping up everywhere) etc.