To say that a believer is "not under law, but under grace" is not to say he has no law to live by. When Jesus did away with the Law of Moses, He did not do away with the eternal moral principles it had taught. These principles had existed forever, and the Law of Moses was simply used by God to reveal them to mankind. "Not under law" means that the Christian is not under the penalty of law, where even one transgression of God's moral laws brought death.
To say that God's laws are being written on the believer's heart by the Holy Spirit, is not to say that each individual must determine for himself what is moral, and what is love. (Morality refers to relationships, whether it be with our Creator, or with one another). It is so easy for us to believe we are acting out of love (i.e. morally), when we really are not. The law being written on our hearts speaks of true obedience motivated by the inexpressible mercy of God and the forgiveness of His Son.
There is a clear, definite and unchanging standard of morality by which the Christian must live. These moral laws speak primarily of our relationship with God, and secondarily of our relationship with one another. Jesus lived under the Law of Moses, but that was no problem, because as God's living Word nothing He said or did contradicted the moral laws of the Old Testament. In fact, Jesus confronted the Jews with ignoring the true moral principles of the Law of Moses. We will never understand what our Saviour is talking about, until we understand the Law of Moses well enough to recognize the abuses He was addressing while on earth.
As an example, let us look at what Jesus said in Matthew 5:33-37:
"Again, you have heard that the ancients were told, 'You shall not make false vows, but shall fulfill your vows to the Lord; but I say to you, make no oath at all; either by heaven, for it is the throne of God..nor shall you make an oath by your head, for you cannot make one hair white or black. But let your statement be, 'Yes, yes' or 'No, no' and anything beyond this is of evil."
The Law of Moses demanded that the Jews keep their oaths made to God, to teach the importance of honesty and commitment before the Lord. The Jews tried to get around the Law by swearing by heaven, or their head, instead of by God, so that they might break their oaths. Jesus said not to play games with God because heaven is God's footstool and God is in control of our very lives. Our honesty and commitment should be such that everyone can trust our word, without further embellishment's being necessary. This is not a change in God's moral law as some have supposed (and have concluded that it is now wrong to take an oath in a court of law). God's moral laws do not change from one time to another; they are simply a reflection of God's holy nature. Hebrews 6:16 says: "For men swear by one greater than themselves...in the same way God...interposed with an oath." The writer to the Hebrews sees nothing wrong with men taking oaths in the name of God.
Jesus' healing on the Sabbath often gives people problems. Some have argued that He was transgressing the letter of the law in order to obey the higher principle of love. Others argue that as Lord of the Sabbath He had the right to not keep this law. Neither of these interpretations is correct. The Sabbath commanded the people, servants and animals to rest from their daily toil and labors. This was not an end in itself, but a time to glorify the Lord of all Creation. The people were to rest in order to worship and serve their Lord. Daily toil and cares had to be put aside, but it was never unlawful to help others (or pluck a few grains of wheat) on the Sabbath. The priests busy with the temple services were not an exception to the Sabbath law, they were keeping its very essence. In the same way, Jesus' healing on the Sabbath was in accord with the true spirit of the Sabbath rest. He was busy in God's service, His every thought was only to glorify the Lord. Jesus transgressed neither the letter nor the spirit of the Sabbath law. The letter of law, understood properly, teaches the higher principle of love. (There is a ceremonial element to the Sabbath law. We are concerned here only with its moral element).
Jesus did not come to bring a new moral code, but to demand obedience to the moral laws taught by the Law of Moses. Yet, rarely is a serious study ever made of the Law of Moses. We tend to skim over the "old" laws, without recognizing their far-reaching implications. The Ten Commandments themselves encompass every moral principle revealed by God to man, and we spend our time arguing against them! The judgements give case by case studies of the application of these principles.
There are things addressed in the Law of Moses which are never mentioned in the New Testament scriptures. They don't have to be. God never intended for the New Testament scriptures to supplant the Old. They must be read together as a whole. Is it wrong for a man to marry his sister, or a first cousin? The New Testament never says so. Should a woman who has been married twice, divorce and reconcile to her first husband? The Law of Moses forbids it. Does God have much to say about the sanctity of the life of the unborn? He does in the Old Testament. Does God approve of Capital Punishment? The life of adulterers and other capital offenders apparently could be redeemed under the Law. Such redemption was specifically and strictly forbidden for premeditated murder. Does 'turning the other cheek' mean we must allow dangerous criminals to enter our homes and threaten our families? Under the Law, a man defending his home was not held responsible for the death of a burglar. The moral principles taught by the Law were never contradicted by Jesus. He taught us to look inward to the real motivations of our hearts, and to fulfill the moral principles of the Law in spirit and truth.
Only when we honor the absolute and unchanging rightness of God's moral law will we be able to recognize what real love is all about. Only when we depend upon the forgiveness of Christ will we have the power to obey when times get tough.