Romans 3:21-23 // All Have Sinned

Based on Romans 3:21-23

21 But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets; 22 Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference: 23 For all have sinned, and come short of the glory of God;

The coming of Christ was predicted by the law and the prophets, along with many aspects of his life and ministry. These give evidence that Jesus was really the Christ, the older scriptures give evidence that the newer scriptures are true. The law itself can only have effect on one that reads, hears or thinks about it, the law is dependent on human interest to be effective. For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh (Romans 8:3). The law cannot save because of human weakness, Jesus manifested/demonstrated the righteousness of God (obedience to the law), which we can also have by faith. Jesus Christ has won this ability to have saving faith for us, it is the kind of faith we need in order to trust God when everything seems to be going against faithfulness. This faith is available for all that believe the word of the gospel, the same, regardless of ethnicity or class. There is no difference between Jews and Gentiles, cultural Christians and profligates where the gospel is concerned; because all have sinned and come short of the glorious standard of character and behaviour that God has detailed in his law.
I find it interesting to read this verse in context now, I have always heard it used to say that every single individual person has sinned, including unborn infants, the mentally disabled and... Jesus? Obviously it can't be taken to mean every person ever without exception, because Jesus has to be the exception. It is interesting that in context it is talking about how Jews and Gentiles (verse 9) are both sinful and need a Saviour. So although it is generally true that all have sinned, the context is addressing the issue of whether the Jews are the exceptions who have not sinned. It is saying that Jews (and cultural Christians) are sinners too, it is not specifically discussing those that do not have the mental capacity for faith. I am not saying that sin does not affect us at every mental state (I think it does) but rather that this verse should not be taken as the final word on this issue.
When it comes to those who are still-born or mentally disabled, it would be best to follow the advice God gives us in 1 Corinthians 4:5 that we should not try to determine people's salvation before judgment is given to us (Dan 7:22; Rev 20:4). God is just, and will not condemn any man for any sin but his own (Deut 24:16). The one who has never known anything is better off than the average sinner (Eccles 6:5). One believing parent makes all his or her children holy, (1 Cor 7:14) we know they do not become well behaved just because of their parent's conversion, so what does it mean that they are made holy? If you believe that God cannot destroy the soul (Mat 10:28) and therefore (of necessity) consciousness (which is part of the soul) you will struggle to reconcile these things. Some may be saved by the faith of those who love them, others may be as if they never knew any thing.

Jesus life is our perfect example of how to live, read about his life in the Bible, reflect on it through the day. You need it more badly than you think.