scripturecatholic.com/baptism.html#baptism-V
Furthermore, baptism as explained by Paul I believe in the scriptures is an archtype for circumsicsion which was performed on the child at 8 days old as a sign that they were entered into the old covenant. Baptism thus, was always meant for infants as their initiation into the new covenant. You are simply dealing with people who do not really understand the covenants. The early Church is clear that baptism is essential.
“But in respect of the case of the infants, which you say ought not to be baptized within the second or third day after their birth, and that the law of ancient circumcision should be regarded, so that you think one who is just born should not be baptized and sanctified within the eighth day…And therefore, dearest brother, this was our opinion in council, that by us no one ought to be hindered from baptism…we think is to be even more observed in respect of infants and newly-born persons…” Cyprian, To Fidus, Epistle 58(64):2, 6 (A.D. 251).
“Likewise, whosoever says that those children who depart out of this life without partaking of that sacrament shall be made alive in Christ, certainly contradicts the apostolic declaration, and condemns the universal Church, in which it is the practice to lose no time and run in haste to administer baptism to infant children, because it is believed, as an indubitable truth, that otherwise they cannot be made alive in Christ.”
Augustine, Epistle 167,7,21 (A.D. 415).