I was asked in the comments of another post to give the "magic dividing line" between what actions not mentioned directly in scripture that I think we can condemn (the example given was masturbation by the commenter, accurately assuming I would condemn it) and those that I don't think we can give a blanket condemnation of (such as contraception).
What follows is my response - with additional commentary to follow:
Here is the dividing line. The question isn't just a matter of action - what action is allowable or not allowable, and if it is allowable then I can do it. The question is "why" something is done.
Is sex permitted? Depends upon why it is done. If it is in the context of marriage and for the benefit of the spouse, it is a good and God pleasing thing. If it is outside of marriage, it is sin.
Is killing permitted? Depends on why the killing is done. Is it in the context of just war, or self defense, or defense of the neighbor... then it may be done. Is it done out of anger, or a desire for revenge for some slight? Then it is a sin.
Is preaching permitted? It depends on the context of the preaching. If one is called and ordained to preach, then it is good. If one usurps the office, it is sin.
What you are falling to see is this - actions do not take place in the abstract. They are not fundamentally good or bad - they are neutral. They are permissible, but not necessarily profitable. The profitablity depends entirely upon the context.
Your approach is trying to define certain actions as intrinsically good or intrinsically bad. No.
"Well, when is idolatry good?" Idolatry is just worship that is in the wrong context to the wrong place. Worship is a good thing if God is worshiped according to His command. Worship is a bad thing if it is to something other than God -- and we call this idolatry.
Thus, I will say even with Contraception, it in and of itself is neither fundamentally good or bad. Tell me the context, tell me the why. Is there an honest concern for ones spouse - so be it. Care for your spouse, as God commands. Does it derive from a desire for greed or selfishness? Then no, that is not good.
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Now the additional comments.
One of the things we do as humans is try to justify certain actions - to set up a fence and say, "ah, these things I can do, and if I do them I know that they are good." This is a false, misleading dream. Because of the wickedness of man, anything, every good blessing which God gives can be turned into sin.
There is no magic line that makes some actions fundamentally good and free from wickedness - actions that are safe and I know that if I just do them, then I am doing good. You are a sinful, fallen being, and you can turn and twist anything, anything into sin.
Thus - the question must always be this - why is something being done. That should be the grounds upon which it is judged.
And you want to know the really, really harsh truth - the one that no one likes? Everything, every act you do is sin.
Tomorrow - when I preach a sermon and conduct worship service - I will be sinning. Why? Because even though I am called, and it is proper and licit for me to be there, I am a sinful being, and part of me will have sinful, ulterior motives. I will want people to respect and praise me for my good job... and that is sin. That's why I pray before the sermon that it be not to my praise... but part of me will want it.
Everything I do is sin, because every single moment I am in this flesh, I am a sinner. Through and through.
This is why Luther said, "Sin boldly." It's not that one does not care about what is right or wrong... no, strive to do the good, strive to act with good and God pleasing motivation... but know that whatever you do, it is sin. And sometimes, the old adam with its false motivations take the fore... so be it. Strive to beat it down and live... sin boldly... with boldness strive against your self, knowing that what you do will still be none the less sinful and worthy of nothing but damnation.
Sin boldly, live your life which will always be nothing but tainted with sin in a bold struggle against sin, but believe more boldly still. Put not your trust in your actions, but trust in Christ Jesus and His redemption.
And pray as you always need to pray - I believe, help my unbelief.