Can a religious 501c3 force staff to attend re3ligious services

No. Can a not-for-profit hire and fire under the laws of the State where it's located? Yes.

It's employment at will, so a person could, in theory, be fired for not attending religious services. Would a discrimination lawsuit be filed? Would the employer be found guilty of discriminating? Possibly.

No they can not force you but they could terminate you if they feel like it. Most state are "at will" employment states. Barring an employment contract they could let you go. Can you give more info?