“For by this grace He engrafts into His body even baptized infants, who certainly have not yet become able to imitate any one. As therefore He, in whom all are made alive, besides offering Himself as an example of righteousness to those who imitate Him, gives also to those who believe on Him the hidden grace of His Spirit, which He secretly infuses even into infants.”
Saint Augustine, On the merits and forgiveness of sins 1:9 (A.D. 412)