You say somewhere that baptism is as a ship. How do you mean that?

Luther:

This ship is strong and indestructible. The passengers are those who are on the way to the harbour, that is to the fulfilment of what God has promised in the sacrament. But many people jump overboard in the sea, so that they drown. Then, there are those who do not believe in the promise anymore and who fall in sin. But the ship itself always remains intact and always holds it’s course.

When someone can return to the ship by God’s grace, he does not need a board to hold on to [the sacrament of confession], but he has to go back in the sturdy ship: by faith back to God’s steady and certain promise.

It is as Peter says in his second letter, the first chapter. He reproves sinners because they have forgotten the cleansing of their former sins. He means thereby that their ingratitude to their baptism and he reprimands their godless unbelief.