Dear friends, if our hearts do not condemn us, we have confidence before God and receive from him anything we ask, because we obey his commands and do what pleases him. And this is his command: to believe in the name of his Son, Jesus Christ, and to love one another as he commanded us. Those who obey his commands live in him, and he in them. And this is how we know that he lives in us: We know it by the Spirit he gave us. (I John 3:21-24,NIV)
So I say, live by the Spirit, and you will not gratify the sinful nature. For the sinful nature desires what is contrary to the Spirit, and the Spirit what is contrary to the sinful nature. They are in conflict with each other, so that you do not do what you want. But if you are led by the Spirit, you are not under law.
The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God.
But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law. Those who belong to Christ Jesus have crucified the sinful nature with its passions and desires. Since we live by the Spirit, let us keep in step with the Spirit. Let us not become conceited, provoking and envying each other. (Galatians 5:16-26,NIV)
Do not be deceived: God cannot be mocked. A man reaps what he sows. The one who sows to please his sinful nature, from that nature will reap destruction; the one who sows to please the Spirit, from the Spirit will reap eternal life. Let us not become weary in doing good, for at the proper time we will reap a harvest if we do not give up. Therefore, as we have opportunity, let us do good to all people, especially to those who belong to the family of believers. (Galatians 6:7-10,NIV)
Christianity is not Judaism, even though we use the definitions which are originally found in Judaism for words such as 'sexual immorality'. Proper behavior in Judaism is governed by laws and commandments which were designed for a specific society that was to remain seperate from all other societies. Proper behavior in Christianity is governed by our willingness, and even our longing, to better the condition of all around us, both Christians and nonbelievers. We are governed not by laws, but rather by motivations. The laws that we pay attention to, such as those forbidding murder, theft, adultery, and perjury, are not exclusive to either Judaism or Christianity; societies all over the world teach their citizenry to obey these same laws.
Our code of morality is not weaker than that found in Judaism; it's stronger. And that is due to its being necessary for us to have the correct motivations as the impetus for our actions, rather than our simply 'going through the motions'. If the motivations for our actions don't conform to what God wants those motivations to be, namely, 'the fruit of the Spirit', then our actions have no assurance of conforming to what he wants of us. Laws can be manipulated, or even twisted so as to appear to support actions that are clearly evil; motivations can't. So it is those motivations which are 'the laws written on the heart'.