Faith is, indeed, necessary to the use of baptism, with this distinction. Actual faith is required in adults, and an inclination to faith in infants. There are, therefore four terms in this syllogism, or there is a fallacy in understanding that as spoken particularly, which must be understood generally. Those who do not believe, that is, who have no faith at all, neither by profession nor by inclination, are not to be baptized. But infants born to believing parents have faith as to inclination.

…We also deny the minor proposition; for infants do believe after their manner, or according to the condition of their age; they have an inclination to faith. Faith is in infants potentially and by inclination, although not actually as in adults. For, as infants born of ungodly parents who are without the church, have no actual wickedness, but only an inclination thereto, so those who are born of godly parents have no actual holiness, but only an inclination to it, not according to nature, but according to the grace of the covenant. And still further: infants have the Holy Ghost and are regenerated by him. John the Baptist was filled with the Holy Ghost, even from his mother’s womb, and Jeremiah is said to have been sanctified before he came out of the womb (Luke 1.5; Jeremiah 1.5). If infants now have the Holy Ghost, he certainly works in them regeneration, good inclinations, new desires, and such other things as are necessary for their salvation, or he at least supplies them with everything that is requisite for baptism, according to the declaration of Peter, “Can any man forbid water to them who have received the Holy Ghost as well as we.” It is for this reason that Christ enumerates little children amongst those that believe, saying, “Whoso shall offend one of these little ones which believe in me.” Inasmuch now as infants are fit subjects for baptism, they do not profane it as the Anabaptists wickedly affirm.

[Zacharias Ursinus was the primary author of the Heidelberg Catechism]