Martyn Lloyd Jones gives a simple test to evaluate whether you really believe what the Bible teaches about justification:

“Before man can be reconciled to God, before man can know God, this sin of his must be removed. God has sad that He will punish sin, and that the punishment of sin is death and banishment from the face of God. This has to be dealt with. And what has happened? Well, says Paul, God has set Him forth as a propitiation. That is the means which God has employed. His being the propitiation for our sins means that God has made Him responsible for our sins. They have been placed upon Him and God has dealt with them and punished them there, and therefore because He has punished out sins in Christ in His body upon the Cross, He can justly forgive us. You see this is high doctrine. It is a daring thing for the Apostle to say, but it has to be said and I repeat it. God, because He is righteous and holy and eternal, could not forgive the sin of man without punishing it. He said He would punish it, so He must punish it, and, blessed be His Name, He has punished it. He is just, therefore, and the justifier of them that believe in Jesus. The sin has been punished, so God, who is just and righteous, can forgive sin. How then does it work? It works like this. God accepts this righteousness of Christ, this perfect righteousness face to face with the law which He honoured in every respect. He has kept it, and given obedience to it, He has borne its penalty. The Law is fully satisfied. God’s way of salvation, says Paul, is that. He gives to us the righteousness of Christ. If we have seen our need to go to God and confess it, God will give us His own son’s righteousness. He imputes Christ’s righteousness to us who believe in Him, and regards us as righteous, and declares and pronounces us to be righteous in Him. That is the way of salvation, the Christian way of salvation, the way of salvation through justification by faith. So that it comes to this. That I see and I believe and I look to nothing and to no one except to the Lord Jesus Christ. I like Paul’s way of putting it. He asks: ‘Where is boasting then? It is excluded. By what law? Of works? Nay, but by the law of faith’. You foolish Jews, says Paul, you are boasting about the fact that you have been circumcised, that you have the oracles of God and that you are God’s people. You must cease to do that. You must not rest upon the fact that you have this tradition and that you are children of your forefathers. There is no boasting, you have to rest exclusively upon the Lord Jesus Christ and His perfect work. The Jew is not superior to the Gentile in this respect. ‘All have sinned and come short of the glory of God.’ We look to Christ and to Christ alone, and not to ourselves in any respect whatsoever.

To make it quite practical let me say that there is a very simple way of testing yourself to know whether you believe that. We betray ourselves by what we say. The Lord Himself said we should be justified by our words, and how true it is. I have often had to deal with this point with people, and I have explained the way of justification by faith and told them how it is all in Christ, and that God puts His righteousness upon us. I have explained it all to them and then I have said: ‘Well, now are you quite happy about it, do you believe that?’ And they say ‘Yes’. Then I say: ‘Well then are you now ready to say that you are a Christian.’ And they hesitate. And I know they have not understood. Then I say: ‘What is the matter, why are you hesitating?’ And they say: ‘I do not feel that I am good enough’. At once I know that in a sense I have been wasting my breath. They are still thinking in terms of themselves; their idea still is that they have to make themselves good enough to be a Christian, good enough to be accepted with Christ. They have to do it! ‘I am not good enough.’ It sounds very modest, but it is the lie of the devil, it is a denial of the faith. You think that you are being humble. But you will never be good enough; nobody has ever been good enough. The essence of the Christian salvation is to say that He is good enough and that I am in Him! As long as you go on thinking about yourself and saying; ‘Ah, yes, I would like to, but I am not good enough; I am a sinner, a great sinner,’ you are denying God and you will never be happy. You will continue to be cast down and disquieted in your soul. You will think that you are better at times and then again you will find that you are not as good as you thought you were. You read the lives of the saints and you realize that you are nowhere. So you keep on asking: ‘What can I do? I still feel that I am not good enough’. Forget yourself, forget all about yourself. Of course you are not good enough, you will never be good enough. The Christian way of salvation tells you this, that it does not matter what you have been, it does not matter what you have done. How can I put this plainly? I try to say it from the pulpit every Sunday because I think it is the thing that is robbing most people of joy of the Lord. It does not matter if you have almost entered into the depths of hell, if you are guilty of murder as well as every other vile sin, it does not matter from the standpoint of being justified with God. You are no more hopeless than the most respectable, self-righteous person in the world. Do you believe that?”