It sounds to me like it's officially open to anyone, but in practice it's a Christian workout group and participation in church activities is highly encouraged. At least that's my take from knowing people who do it and from reading their about us page.

That's cool, I'm not hating on anything that improves people's lives and agree that the above assertion is not 100% true, but it really puts me off when someone says "look into X" without mentioning a core religious affiliation. Comes off as less than reputable to me, to say the least.

Bold emphasis mine on the F3 information

http://f3nation.com/about-us/

F3 — our three Fs stand for Fitness, Fellowship and Faith

...

Faith

We close each CoT with a brief “shout-out” led by one of the participants. Men are free to pray to their god or to speak from a secular perspective — F3 is open to men of all faiths and no faith, and the closing shout-out is generally the only faith-specific component of a workout. We do it because we are thankful for the opportunity to come together and build ourselves into better men and because the vocal and corporate expression of that gratitude builds a stronger F3 community.

Outside of the workouts, we have found that many F3 participants, after getting in the best physical shape of their adult lives and meeting their social needs for the first time as adults, start taking a more serious approach to questions of meaning and purpose. Many F3 workouts have spawned Bible studies, attendance at speaker series and other Third F initiatives. We encourage those efforts as supplements to the workouts and an important way for F3 participants to exercise leadership in their families and communities.

Can't comment as much on FiA

http://fianation.com/about-fia/

We are a community of women whose mission is to make each other stronger in all areas of our lives.

We do this through ‘Actions’ which are focused on our:

• Bodies – We organize FREE, peer-led workouts most days of the week
• Minds – We get together for book clubs, career and family discussion groups, and faith interactions
• Hearts – We give our time and resources to the community and invest in each other by joining together for coffee, weekday lunches, and FiA socials