First, just because a doctrine brings you comfort or the warm fuzzies, doesn't mean it is true. I could reason that denying the existence of hell brings me comfort. How horrible it is to think of all those people forever in hell suffering for their sins. How cruel of you to say that God won't save everyone. The doctrine of hell is not pleasant, but it is still true. And we need to know the cold hard truth if we ever want to find real comfort in Christ. The same is true for other truths as well. So using feelings as an argument for a doctrine is not valid. There must be real truth to back up the comfort. The covenant of grace argument lacks any scriptural support and so it can give only false comfort.
Even if in spite of evidence that the covenant of grace is not true, you still believe it and embrace the comfort that your children are saved for a time, then you are still left with a hard question. What about the children of unbelievers when they die in infancy? Does God send them to hell? Does God send mentally handicapped children to hell who can't understand the gospel or any truth about God? What about some who had babies die before they were saved, are those children in hell? So I ask you, how would you explain that?
There are two ways in which infants are unique from the rest of mankind. One, they cannot exercise faith. Faith requires the ability to understand the truth, to accept the truth, and act upon the truth. Since infants and the mentally handicapped are mentally unable to do this, they cannot believe the gospel. Faith comes from hearing--this is hearing and understanding. If they cannot have faith they cannot believe the gospel that gives eternal life. Two, they cannot comprehend the evidence for God in his creation. In Romans 1:19-20 we learn that God has clearly revealed Himself to this world in the creation. verse 20 says, 'For His invisible attributes, namely his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse." But it seems plain that infants and the mentally handicapped cannot see God's attributes because they do not have the mental capabilities. It seems logical then that they would have an excuse before God. They cannot see and understand the evidence that God has left to leave everyone without excuse in the day of judgement. They cannot reject something they don't understand,
God knows everything. He knows from the beginning of the world all the people who will exist in the history of the world. He knows exactly how many days each person will live. He knows how and when they will die. "Your eyes saw my unformed substance; in your book were written, every one of them, the days that were formed for me, when as yet there was none." Psalm 139:16. So before the world began, God knew exactly how many children would die in infancy and how many mentally handicapped people would be born before He made the world. God doesn't give salvation to all or some children only to take it away when they reach some level of accountability. If God were to save infants and the mentally handicapped, then He would have to elect them before the foundation of the world.
Infants, even newborn babies are not free from sin and the guilt that comes from Adam's sin. David knew this, for in Psalm 51:5 he said, "Behold, I was brought forth in iniquity, and in sin did my mother conceive me." David knew that the moment he was conceived he had a sinful nature. He came out of the womb with a heart that was rotten to the core. Also Romans 5:12 teaches us that we all died spiritually in Adam. "Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned." Adam was our representative and when he fell into sin, so did we. His spiritual death spread to all people and so all people have become guilty. All this teaches us that even babies and the mentally handicapped are sinners and are guilty before God. If God were to save those that die in infancy, He would have to apply the forgiveness and righteousness of Christ to them.
God understands there is a difference between those who are infants and those who are not. Deuteronomy 1:39 describes infants as those who "have no knowledge of good or evil." Or consider Isaiah 7:16, "For before the boy knows how to refuse the evil and choose the good...." In both these passages God describes very young children as those who don't choose to sin with a full understanding of what they are doing. They sin because they are sinners and have a sin nature. Later as they grow older they will deliberately and willfully choose sin rather than good. Children are probably more quickly accountable before God than we think. And only God knows when a child can choose good or evil. So we must from an early age teach them the gospel.
God is silent about the salvation of infants and the mentally handicapped. There are no clear texts which we can go to. There is one that is often referred to which could help but I think it is still unclear. In 2 Samuel 12:23, David says after the death of his son, "I shall go to him, but he will not return to me." There are two other phrases like this. One in 1 Samuel 28:19 where Samuel says to Saul that he and his "sons will be with me." This doesn't refer to Saul and sons joining Samuel in heaven since Saul was no believer, rather it says that Saul and his sons will join Samuel in the grave or Sheol. In Genesis 37:35, Jacob says, "No I shall go down to Sheol to my son, mourning." when he learns that Joseph has "died". While David's statement may refer to his hope of a reunion in heaven with his dead son, there is also good reason to believe David simply means that he will join his son in Sheol or the grave. We must trust that God has a good reason for this silence on the salvation of infants.
The best hope and comfort we have is found in the character of God. God gives us all the comfort we need in Himself. When Job could not understand why all these troubles were coming into his life, he found his comfort in the Lord who is wise, powerful, sovereign, and loving. In Psalm 119:68 we read, "you are good and do good." God is good and everything He does is good. There is nothing mean, cruel, or bad in God. God is perfect in justice. Abraham said, "Shall not the Judge of all the earth do what is just?" Genesis 18:25 And so He is; God is always right. He never errs. He always brings perfect justice. We need to trust Him, especially in the things He has not revealed to us.
When I consider all that scripture teaches on this subject. I believe it is reasonable to be assured that God will elect and save all those who are mentally unable to believe the gospel when they die. It would seem consistent with scripture and God's nature. But in the end, we need to trust that God will do what is good and right in this matter. Trust Him, when we get to heaven this will all make sense and we will rejoice to see that yes, indeed, God did what was best and right.
* The verse commonly used to teach that God gives salvation to the children of believers by virtue of the covenant of grace is found in Genesis 17:7. "And I will establish my covenant between me and you and your offspring after you throughout their generations for an everlasting covenant." There are two good reasons this verse has nothing to do with the salvation of the children of believers.
1) "Your offspring" or descendants refers to the physical offspring of Abraham. From one generation of Jews to another, God will keep His covenant with them. This is also shown in the very next verse where God says that He will give to Abraham's offspring the land of Canaan, the very land that Abraham sojourned through his life.
2) "to be God to you and to your offspring after you" and "I will be their God." means God will be Israel's God in a national sense. This use of this phrase has little to do with salvation. God often said that He was Israel's God and she was His people, even when most of the people were corrupt and godless. To read in this expression a salvific sense is to put a New Testament understanding into an Old Testament expression while at the same time ignoring the context.