'Your secret is safe with me.'...?

I assume it was "I love you", though it actually took me a second of confusion over "wait, what secret?".

However, earlier in the film the line is used to hint Atom "understands" Max, and there's the very odd scene of Atom looking into the mirror that otherwise is unconnected with the rest of the film thematically and is never picked up on again.

Because of that, I kinda wanted the "secret" line at the end to be "Your mother's soul was inside Atom all along". :)

Me too, any serious answers on here?

I was confused about that also. Maybe the kid knew that his dad really loved him but wanted to keep it a secret? and he didn't want anyone to know that?

I agree with everyone claiming Charlie loved his own son

As for why that's a "secret," I think it's just a joke -- a father shouldn't be afraid to admit he loves his own son, but Charlie's kinda messed up and can't bring himself to say it.

Max is saying, in effect, "Yes, I know you love me and that you're afraid to admit it, so don't worry I won't tell"

And of course it's a callback to Max saying the same thing to Atom. Not that Atom loves him, but that Max is just a very understanding and forgiving person

In other words, it's about Max, not Charlie or Atom