Now, although I may not be able myself to refute the arguments of these men, I yet see how necessary it is to adhere closely to the clearest statements of the Scriptures, in order that the obscure passages may be explained by help of these, or, if the mind be as yet unequal to either perceiving them when explained, or investigating them whilst abstruse, let them be believed without misgiving. But what can be plainer than the many weighty testimonies of the divine declarations, which afford to us the dearest proof possible that without union with Christ there is no man who can attain to eternal life and salvation; and that no man can unjustly be damned,--that is, separated from that life and salvation,--by the judgment of God? The inevitable conclusion from these truths is this, that, as nothing else is effected when infants are baptized except that they are incorporated into the church, in other words, that they are united with the body and members of Christ, unless this benefit has been bestowed upon them, they are manifestly in danger of damnation. Damned, however, they could not be if they really had no sin. Now, since their tender age could not possibly have contracted sin in its own life, it remains for us, even if we are as yet unable to understand, at least to believe that infants inherit original sin.And therefore, if there is an ambiguity in the apostle's words when he says, "By one man sin entered into the world, and death by sin; and so it passed upon all men;" and if it is possible for them to be drawn aside, and applied to some other sense,--is there anything ambiguous in this statement: "Except a man be born again of water and of the Spirit, he cannot enter into the kingdom of God?" Is this, again, ambiguous: "Thou shalt call His name Jesus, for He shall save His people from their sins?" Is there any doubt of what this means: "The whole need not a physician, but they that are sick?" --that is, Jesus is not needed by those who have no sin, but by those who are to be saved from sin. Is there anything, again, ambiguous in this: "Except men eat the flesh of the Son of man," that is, become partakers of His body, "they shall not have life?" By these and similar statements, which I now pass over, --absolutely clear in the light of God, and absolutely certain by His authority,--does not truth proclaim without ambiguity, that unbaptized infants not only cannot enter into the kingdom of God, but cannot have everlasting life, except in the body of Christ, in order that they may be incorporated into which they are washed in the sacrament of baptism? Does not truth, without any dubiety, testify that for no other reason are they carried by pious hands to Jesus (that is, to Christ, the Saviour and Physician), than that they may be healed of the plague of their sin by the medicine of His sacraments? Why then do we delay so to understand the apostle's very words, of which we perhaps used to have some doubt, that they may agree with these statements of which we can have no manner of doubt? (St. Augustine, On the Merits and Forgiveness of Sins, And Infant Baptism, 3:8-9)
***The Scriptures are clear that the unjustified are damned.***
I don't think anyone who believes the Bible would argue that God allows the unjustified into heaven. But is seems, based on references in Scripture and Jesus on stated intentions towards children, that there must be a special case allowance to justify those who are utterly incapable of meeting the requirements on their own.
How else was this child in the following passage saved (for we know that David isn't in hell)?
"And the Lord afflicted the child that Uriah's wife bore to David, and he became sick. David therefore sought God on behalf of the child. And David fasted and went in and lay all night on the ground. And the elders of his house stood beside him, to raise him from the ground, but he would not, nor did he eat food with them. On the seventh day the child died. And the servants of David were afraid to tell him that the child was dead, for they said, "Behold, while the child was yet alive, we spoke to him, and he did not listen to us. How then can we say to him the child is dead? He may do himself some harm." But when David saw that his servants were whispering together, David understood that the child was dead. And David said to his servants, "Is the child dead?" They said, "He is dead." Then David arose from the earth and washed and anointed himself and changed his clothes. And he went into the house of the LORD and worshiped. He then went to his own house. And when he asked, they set food before him, and he ate. Then his servants said to him, "What is this thing that you have done? You fasted and wept for the child while he was alive; but when the child died, you arose and ate food." He said, "While the child was still alive, I fasted and wept, for I said, 'Who knows whether the LORD will be gracious to me, that the child may live?' But now he is dead. Why should I fast? Can I bring him back again? I shall go to him, but he will not return to me."
"...I shall go to him, but he will not return to me."