There is an assumption here. The assumption being that they ONLY did full submersion Baptism.
how can/does the Catholic church justify infant sprinkling ?
1. the infant is not old enough to make the choice on their own
In the Gospels, the children which Jesus saved were too young to seek His aid on their own. Their parents needed to do it for them. And Jesus always healed them on behalf of their parents. (Matthew 15:28
; John 4:50
Why then, would Jesus refuse to save the little children in Baptism? (Matthew 19:14
and 2. the baptism is not submersion–like john the baptist and Jesus.
According to the Tradition of the Catholic Church, all three types of Baptism are permitted.
If you want to interpret the requirement of Baptism according to the letter, then you must be Baptized in the Jordan River. There is no other river mentioned where anyone else was Baptized.
By the way, full submersion is not mentioned either. That is a Protestant reading into Scripture.