Catholics on infant baptism… 1) It’s not just a Catholic thing. Many Protestant denominations also baptize infants, based on the fact that whole households were baptized in Acts (which may or may not have included infants), and Colossians (as others have noted) draws a parallel between it and circumcision. And quite frankly, it has always been done. So 2), the real difference is what people consider to be the effect of baptism. Catholics, basing themselves on scripture and tradition, understand baptism to remit sins. St. Peter says baptism saves (1 Pet 3:21). St. Paul says it washes away sins (Acts 2:38; 22:16), participates in Christ’s death and resurrection (Rom 6:3-4), puts on Christ (Gal 3:27), sanctifies and justifies (1 Cor 6:11), regenerates (Titus 3:5). Furthermore, Jesus makes it an absolute requirement for entrance into the Kingdom (John 3:5).
Obviously, if you are an adult (to whom the Apostolic letters were written), you must also have repented and turned to Christ in faith. Otherwise, your baptism would be a lie. But to a child, who is not excepted from Jesus’ requirement, it is obviously impossible to make a declaration of faith. But it is necessary for that child to grow into the grace he has been given, lest he fall from grace.
Note the symbolism of the whole thing. Christ’s grace is totally gratuitous. There is nothing this child could possibly have done to merit it. And Catholics are still accused, ironically (and not by you), of works righteousness. It is a mystery too deep to fathom.