Unless Infants are Baptized, They Remain in Darkness
“I have come,” says Christ, “a light into the world, that whosoever believes in me should not abide in darkness.” Now what does this passage show us, but that every person is in darkness who does not believe in Him, and that it is by believing on Him that he escapes from this permanent state of darkness? What do we understand by the darkness but sin? And whatever else it may embrace in its meaning, at any rate he who believes not in Christ will “abide in darkness,”— which, of course, is a penal state, not, as the darkness of the night, necessary for the refreshment of living beings. [XXV.] So that infants, unless they pass into the number of believers through the sacrament which was divinely instituted for this purpose, will undoubtedly remain in this darkness.
Source: Merits and Remission of Sin, and Infant Baptism (New Advent)