This is a difficult question to answer, but one you hear quite often, so I thought I would spend some time trying to figure it out. I hope what I have learned will bring some comfort to someone.
First let's look at how God passes judgement:
Revelation 20:11 Then I saw a great white throne and him who was seated on it. From his presence earth and sky fled away, and no place was found for them. 12 And I saw the dead, great and small, standing before the throne, and books were opened. Then another book was opened, which is the book of life. And the dead were judged by what was written in the books, according to what they had done. 13 And the sea gave up the dead who were in it, Death and Hades gave up the dead who were in them, and they were judged, each one of them, according to what they had done. 14 Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire. 15 And if anyone's name was not found written in the book of life, he was thrown into the lake of fire.
In the above passage those written in the book of life were sparred, and those not written in the book of life were cast into the lake of fire.
Those not written in the book of life are judged and condemned by their on works.
So now we have to determine how someone gets in the book of life and what are the works they are judged by.
I can show you scripture upon scripture telling you that you must be reborn or that you must come by faith and repentance, but we are dealing with people that cannot do that. Babies or the mentally disabled will not have the ability or opportunity to come by faith and repentance. So could babies possibly get in the Lambs book of life?
Well I went to my favorite Bible study website and did a search on 'book of life” and found these two verses....
Revelation 17:8 The beast that you saw was, and is not, and is about to rise from the bottomless pit and go to destruction. And the dwellers on earth whose names have not been written in the book of life from the foundation of the world will marvel to see the beast, because it was and is not and is to come.
Revelation 13:8 and all who dwell on earth will worship it, everyone whose name has not been written before the foundation of the world in the book of life of the Lamb who was slain.
So those that are written in the Lambs book of life were written there before they were born, before they did anything good or bad. Strictly by God's sovereign decree.
For more on Election click here.
Ephesians 1:4 even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love
So now lets look at the second part of our question...
What are the works they(those who are condemned to hell) are judged by?
Luke 10:16 The one who hears you hears me, and the one who rejects you rejects me, and the one who rejects me rejects him who sent me.
Rejecting Christ is the ultimate sin, the one that cannot be forgiven. But it is also one that babies or the mentally disabled cannot commit. Nor can they themselves commit any other sin to be judged by.
Romans 2:14 For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law. 15 They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them 16 on that day when, according to my gospel, God judges the secrets of men by Christ Jesus.
Notice in Romans 2:14-15 above says that Gentiles that do not have the law will be judged by their consciences. There consciences will either accuse them or excuse them. If they acted against what they knew to be right they are accused by their conscience, If their conscience finds no fault in them they are excused. Wouldn't this also apply to those that never had an opportunity to act against their conscience?
My contention at this point is that babies and the mentally disabled die without sinning because they are written in the Lambs book of life.
But to hold this position we must deal with two other problems that others may bring up...
1. Psalms 51:5 Behold, I was brought forth in iniquity,and in sin did my mother conceive me.
We are born with a fallen sinful disposition, in other words we are sinners by nature. But babies have not yet acted on that nature and acquired sins in need of repentance.
Now all we have to worry about on this question is the inherited guilt we receive from Adam. If we have inherited Adams guilt how can a baby be innocent even if they have not sinned? I believe the answer to this is in Christ's atoning work on the cross. Christ paid the price for inherited guilt as well as personal guilt for those who are written in the Lambs book of life. When I came to Christ I repented for my sins and I continue to do so, but I never felt the need to repent of Adams sin only my own.
2. John 14:6 Jesus said to him, I am the way, and the truth, and the life. No one comes to the Father except through Him.
Certainly it is through Christ that anyone is saved. But the answer to number 1 also works for number 2. Christ's work on the cross also covers the inherited guilt from Adam, at least for the elect. And since babies and the mentally disabled are not capable of sinning and cannot be judged for their sin then it is reasonable to conclude that they are elect and they are written in the Lambs book of life.
2 Samuel 12:21 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. 22 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? 23 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.
David certainly believed His child was in heaven and if you have lost a baby or someone with a mental disability that made them incapable of understanding you can see them again if you make your salvation sure.
(If you are not sure of your salvation click here)Other links on this subject:
From RBC.orgFrom the refinersfire.orgFrom Gotquestion.comFrom DesiringGod.orgFrom GTY.org (John Macarthur)