Luther's smaller catechism...
It is not the water indeed that does them, but the word of God which is in and with the water, and faith, which trusts such word of God in the water. For without the word of God the water is simple water and no baptism. But with the word of God it is a baptism, that is, a gracious water of life and a washing of regeneration in the Holy Ghost, as St. Paul says, Titus, chapter three: By the washing of regeneration and renewing of the Holy Ghost, which He shed on us abundantly through Jesus Christ, our Savior, that, being justified by His grace, we should be made heirs according to the hope of eternal life. This is a faithful saying.
In other words without the Word we have only water.
But one can be regenerated by only the Word!
So if all the above is true, why not just speak the sure Word to the infant?
Of his own will begat he us with the word of truth, that we should be a kind of firstfruits of his creatures.
(Jas 1:18)
Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever.
(1Pe 1:23)