What does baptism mean?

Luther:

Two things: baptism and resurrection; and together that is the whole content of justification. When the minister baptizes the child in the water, that means his death. When he lifts it out, then that means life. Paul explains it in Romans 6.

This death and resurrection we call the ‘new creation’, ‘rebirth’ or ‘spiritual birth’. You do not have to understand this allegorically, as if it is about the death of sin and life in grace, like most people do. It points to real death and real resurrection. Otherwise baptism would be a fiction, because sin does not die before and grace is not complete until the body of sin that we drag along in this life is destroyed, as Paul says in the same place. As long as we are in the flesh, there are fleshly desires. So they are not yet dead in this life.

When we make a beginning with faith we also make a beginning with death to this world and the life for God in the future life. Faith is dying and rising, a spiritual baptism, in which we go under and get up again.