Heidelberg Q73

Why then doth the Holy Ghost call baptism the washing of regeneration, and the washing away of sins? God speaks thus not without great cause: not only thereby to teach us, that as the filth of the body is purged away by water, so our sins are removed by the blood and Spirit of Jesus Christ; but especially, that by this divine pledge and sign he may assure us, that we are spiritually cleansed from our sins as really, as we are externally washed with water.