Augustine uses the term “infusion” (like the Council of Trent) and not “imputation” (like Luther and Calvin) when discussing God’s act of justification:
“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)
Not only is this a great quote for showing that Augustine thought of justification in terms of infusion (pouring in), it also shows that Augustine believed that baptism justifies infants ex opere operato. Augustine was not a proto-Lutheran or a proto-Calvinist. He was a Catholic Christian, just like Saint Paul.