John Bunyan tells us in Grace Abounding that he was in that condition and in an agony of soul for eighteen months. The time element does not matter, but any man who is awakened and convicted of sin must be in trouble about this. How can he die and face God? He is aware that he cannot in and of himself, and therefore he is unhappy and troubled. There is no peace; he does not know what to do with himself; he is restless. Having 'peace with God' is obviously the opposite of that. It implies first and foremost that the man's mind is at rest, and he has that rest because he now sees that this way of God, as provided in Christ, is really a way that satisfies every desideratum. Now he can see how this satisfies the justice and the righteousness and the holiness of God. He can see how in this way God can justify the ungodly, as Paul has already put it in [Romans] chapter 4. He thinks it out and he says, 'Yes, I can rest upon that; because God "justifies the ungodly" He can justify even me'.
You notice that I put this intellectual apprehension and understanding first. There is no peace between man and God until a man grasps this doctrine of justification. It is the only way of peace. And it is something that comes to the mind, it is doctrine, it is teaching. In other words we are not just told, 'All is well, do not worry. All will be all right in the end; the love of God will cover you.' That is not the Gospel. It is all stated here, in detail, in this explicit manner; and it comes as truth to the mind. The first thing that happens is that the mind is enlightened, and the man says, 'I see it. It is staggering in its immensity, but I can see how God Himself has done it. He has sent His own Son and He has punished my sin in Him. His justice is satisfied, and therefore I can see how He can forgive me, though I am ungodly and though I am a sinner.' The mind is satisfied.
You will never have true peace until your mind is satisfied. If you merely get some emotional or psychological experience it may keep you quiet and give you rest for a while, but sooner or later a problem will arise, a situation will confront you, a question will come to your mind, perhaps through reading a book or in a conversation, and you will not be able to answer, and so you will lose your peace. There is no true peace with God until the mind has seen and grasped and taken hold of this blessed doctrine, and so finds itself at rest. --
Martyn Lloyd-Jones