BARRY: I've been thinking about your question and the answer come up and hit me — bang. This is my parish. I don't know how much I can do but you're right, Edie — I'll never find out if I don't come down here and take a good look for myself.
By saying that Barry was hiding in the church, Edie provokes him to do the right thing. He decides to get involved in his community, and bring Christian love out into the world. He realizes you can't just preach theory—you need to do.
TERRY: If I spill, my life ain't worth a nickel.
BARRY: And how much is your soul worth if you don't?
Terry is still thinking in terms of the gangsters' code, where snitching is worse than murder. Barry's encouraging him to think in light of eternal morality, and think about what's actually good for the soul.
EDIE: Which side are you with?
TERRY: Me? I'm with me. Terry.
Terry insists that he's just out for himself—he doesn't have any sort of social conscience. But he felt bad when Joey got killed, and that undercuts him. He's not really totally self-centered. He cares.
TERRY: You wanna hear my philosophy of life? Do it to him before he does it to you.
This is more tough guy shtick from Terry. He's stuck in a contradiction here: he feels like killing Joey was wrong, but Johnny "did it to him" (killed Joey) before Joey could snitch.
EDIE: Shouldn't everybody care about everybody else?
TERRY: Boy, what a fruitcake you are!
EDIE: I mean, isn't everybody a part of everybody else?
TERRY: And you really believe that drool?
Edie thinks we should care about each other because we're all interconnected, all a part of life. But Terry is still viewing us as all detached from each other—part of a dog-eat-dog world where everyone's looking out for number one.
CHARLEY: You're getting on. You're pushing thirty. You know, it's time to think about getting some ambition.
TERRY: I always figured I'd live a bit longer without it.
Barry's telling Terry he's got too options: he can keep following true morality, and testify against Johnny instead of trying to assassinate him. Or, he can take his gun, keep following the gangster's code, and get violent revenge on Johnny without actually correcting the bigger problem of the corrupt union dominating the longshoremen.
LONGSHOREMEN: How about Terry? He don't work, we don't work.
JOHNNY: Work! He can't even walk!
LONGSHOREMAN: If Terry walks in, we walk in with him.
The longshoremen should probably just walk in anyway. But they need Terry to demonstrate his principles before they can demonstrate their own. It would be easier if everyone decided to walk in—but, instead, one guy needs to walk in to get everyone else to do it.