The law is man's essential being standing against his existence, commanding and judging it. In so far as his essential being is taken into his existence and actualized in it, the law has ceased to be law for him. Where there is New Being, there is no commandment and no judgement. If, therefore, we call Jesus as the Christ the New Being, we can say with Paul that the Christ is the end of the law.Tillich is trying to show what salvation is. He is taking law in the Lutheran sense, as moral codes and rules that show us how things ought to be (essential being). Existence in life as it is, not as it should be. In Christ and the person that participates in Christ, "essential being is taken into his existence", that is, there is a modification of the personality taken place so that one becomes what one really should be. The Fathers expresses the same thing by separating between the image and likeness of God. The Image of God we have, the likeness is lost because of sin. By God's grace, we can regain that likeness. This is what Tillich calls New Being.
Systematic Theology II, 119.
His point is that moral rules and regulations is part of the existence, that is, a human being that takes part in the New Being has no need for them, because the reality that the rules express is already within that person. (This is another way to express what I tried to say about war here.)
That means that a Christianity that is very much concerned with moral laws and rules, is, well, not very Christian.