It's wrong to lie about being religious when you aren't.
Jonah just starting dating Bryan and found out that Bryan and his family are deeply Christian, though Jonah is Atheist.
Jonah wants to try to make it work with Bryan.
Jonah tells Bryan that he is not religious but is open to gaining a deeper understanding of his Christian beliefs.
Jonah tells Bryan that he is also deeply Christian in an attempt to gain acceptance.
Bryan appreciates Jonah being open minded and is happy to continue dating him without pressuring him to be Christian.
It becomes clear to Bryan over time that Jonah is not Christian at all, and he gets mad at Jonah for lying.