Baptism, then, is a purification from sins, a remission of trespasses, a cause of renovation and regeneration. By regeneration, understand regeneration conceived in thought, not discerned by bodily…’Unless a man be born of water and of the Spirit, he cannot enter into the kingdom of God.’ Why are both named, and why is not the Spirit alone accounted sufficient for the completion of Baptism? Man, as we know full well, is compound, not simple: and therefore the cognate and similar medicines are assigned for healing to him who is twofold and conglomerate:— for his visible body, water, the sensible element — for his soul, which we cannot see, the Spirit invisible, invoked by faith, present unspeakably. ‘For the Spirit breathes where He wills, and you hear His voice, but cannot tell whence He comes or whither He goes.’ He blesses the body that is baptized, and the water that baptizes. Despise not, therefore, the Divine laver, nor think lightly of it, as a common thing, on account of the use of water. For the power that operates is mighty, and wonderful are the things that are wrought thereby.
-Gregory of Nyssa, On the Baptism of Christ