The reason I am so drawn to having this conversation is that it's right where we all live. And it involves that tension I think we all feel between the legalism we were brought up with and our understanding of liberty in Christ, and also the tension between the fact (yes, it is a fact) that our consciences have been cleansed and are now absolutely perfect and guilt free--no, we couldn't possibly be cleaner, purer, or more righteous in God's sight--and the feeling we sometimes have that we are guilty.
And how do we assuage that "guilty" feeling? I think it's often by trying to be more obedient. It's our nature to try to do that. But when we find ourselves seeking relief from the feeling of guilt in obedience to law--any law, OT, NT, some standard we set up in our mind derived from comparing ourselves to others, whatever--it should be a red flag to us that we have forgotten the cross. For however brief a moment, and to whatever degree, when we seek to relieve our guilty feeling through our own performance, we have forgotten His mercy. And that is a very dangerous thing. Because as soon as we forget His mercy to us, we diminish our capacity to show it to others.
Something else that needs to be pointed out whenever we start talking about obeying Old Testament law (even as a wise and practical guide for living), is that Scripture doesn't make any distinction within that code that allows any parceling. So you simply cannot separate a discussion about obedience to any Old Testament law without dealing with the dilemma presented by the fact that Jesus and the apostles never said anything along the lines of, "Ok, guys, here is a list of the ones we are doing away with, and here is a list of the ones that still apply." Christians have those lists in their heads but they are nowhere in Scripture. So if we speak of obedience to certain laws--or to even one of them--"out of gratitude," then how do we avoid advocating the obedience of the entire law out of gratitude?
But now back to motive. If our motive is not to assuage our guilty conscience, since we are truly guilt-free, then what motive do we have to obey--specifically, setting the Old Testament law aside, what motive do we have to abstain from immorality as defined by the New Testament? Certainly, the "law of Christ" can be summed up as 1. Love God and 2. Love others. And if that is all we ever thought about, if our motive was simply love for God and others in response to His love for us, out of thankfulness, then wouldn't "immorality" be a non-issue? When we think of New Testament admonitions toward how we should "walk" as God's people, love as the motive really does cover everything. "We love because He first loved us," and we prove He has loved us when we love one another (1 John).
But there is another "motive" we all tend toward. Or I know I am confronted with this every day. The best way I know to illustrate it is with this familiar passage:
1Co 6:9 Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, 1Co 6:10 Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God. 1Co 6:11 And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God.
Legalists loooooove this passage. The self-righteous simply adore it. Never mind that it condemns them all with the same standard by which they judge others. They can't be bothered with that. They just love to use it to prove that salvation is by works!
Let's face the facts, if it were true that by doing something on that list, you are disinherited from the kingdom, then we are all dead. The cross did nothing for us, and we have no hope. We've all done things on that list since becoming Christians. But the power of the cross and the glory of mercy is stated right there in that passage: even though we still do those things, those things are not who we are.
"And such were some of you. But you are washed." In other words, in God's eyes that is not who you are, anymore. And it has nothing to do with your ability to avoid doing those things. And thank God it doesn't!
What then is my motive for abstaining from those things which I know to be immoral as the Bible defines immorality, and both harmful to myself and others? It certainly is not to assuage my guilt over having done them, or even my guilt for continuing to fall into them. I am free of that guilt because of Christ's righteousness. And because of His mercies new every morning. But when I do those things I still may feel guilty. And such a feeling of guilt or shame , when it doesn't drive me to my knees in thankfulness at the foot of the cross, may instead drive me back to legalism, which always leads to projecting judgment onto someone else. Self-righteousness is the twin brother of guilt, they are truly two sides of the same coin. And both are an affront to the finished work of Christ.
So just as falling into self-righteous law keeping should be a red flag reminding me to not forget the cross; falling into immorality should be a red flag reminding me to not forget the cross. And when I do fall and feel guilty, I need to run to the cross and be reminded that I am forgiven and cleansed. And be restored once again to the joy of that forgiveness so that I can share it with others. And love them. My motive? "The love of Christ compels me." That's the communion we are invited to experience with Him and with one another in Him, whereby we "fulfill the law of Christ."