What does Foster mean when he claims, on page 157, that “The thing about baptism is, you have to be ready to receive it”?

In context, the author is referring to rebirth.... and our willingness to accept or reject changing our lives.

Symbolically, that’s the same pattern we see in baptism: death and rebirth through the medium of water. He’s thrown into the water, where his old identity dies with his older brother. The self who bobs to the surface and clings to the sailboat is a new being. He goes out an insecure, awkward younger brother and comes back an only child, facing a world that knows him as that kid brother, as his old self. The swimming coach can’t stop reminding him how much better his brother was. His mother can’t relate to him without the filter of his brother. Only the shrink and his father can really deal with him as himself, the shrink because he never knew the brother and his father because he just can. Moreover, it’s not just everyone else who has a problem; Conrad himself can’t really understand his new position in the world, since he’s lost some key elements to placing himself in it. And here’s the thing he discovers: being born is painful. And that goes whether you’re born or reborn.