For me it's helpful to understand that baptism is not our sign of commitment to or faith in God, but God's "sign and seal" of His Covenant of Grace to us, as he says in Genesis 17:7: "to be a God unto thee, and thy seed after thee." Thus baptism into the visible church for infants is a way of God signifying that, just as children were circumcised and thus symbolically brought into a set apart community in the Old Testament, so now children of believing parents are brought into the set apart community of the visible church. So the expectation, from a human perspective, is that the child will remain in the visible church and will be saved by grace in the fullness of God's time.
So practically speaking, in terms of their salvation, there is no difference between the infant who has been physically baptized and the infant who has not, because the physical act does not confer salvation. However, the baptized infant is already considered a part of God's visible church according to the Covenant of Grace, so the eventual salvation of that infant (assuming they are elect), is something we expect to happen, since they are already sealed into the visible church, and already are partakers of all its benefits.
So the key difference between baptized infants and unbaptized infants is (in my mind) that baptized infants are physically given the sign of being considered set apart and "holy" (I Corinthians 7:14) as a part of God's church.
Mason
Member, Redeemer Presbyterian Church (PCA)
New York, NY
"Come now, and let us reason together," says the Lord, "Though your sins are like scarlet, they shall be as white as snow; though they are red like crimson, they shall be as wool." - Isaiah 1:18
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