Floyd Mayweather Sr. has his son's back in his beef with Justin Bieber -- saying the pop star unfairly kicked Floyd to the curb just because he found God. 

As we previously reported, Floyd Jr. is furious with his former friend -- who's been distancing himself after some counseling from his church. Bieber even decided to unfollow Floyd on Instagram as part of a bigger plan to back away from people who don't share the same Christian values.

Enter Floyd Sr. ... who says he doesn't really know if the two will ever be able to bury the hatchet.

He also said Justin needs to be true to himself -- but just because he's a godly man now doesn't mean Floyd has to change.

The message is essentially ... Floyd's still the same guy who had your back in the past -- you shouldn't push him away now.

But will Bieber get an invite to the Mayweather vs. McGregor after-party?

"He might," Floyd says ... "It's a maybe."