“… for whatever is not from faith is sin”
What is this verse saying? That any action not motivated and done by complete faith in God is sin. Anytime you do anything you’re not absolutely confident is God’s will for you to do, if you have a doubt about doing that thing, whether the thing in and of itself is right or wrong, you’ve sinned.
A man was in the back bedroom getting dressed. His wife was out in the living room talking with a friend. He yelled out, “Is this shirt clean enough for me to wear?” Without hesitation, she said “No” and went on talking.
After a while he came out, buttoning up another shirt. He asked, “How did you know that shirt wasn’t clean without looking at it?” She said, “If you had to ask, it wasn't.”
That's what God is saying here: whatever you don't have confidence in (i.e., “is not of faith”) is doubtful. Leave it alone.
Often we wonder whether a course of action is right or wrong. Since we don't know absolutely, we do it anyway. But if we think it may be wrong, we should leave it alone until we're sure it's right. We ought to give God the benefit of the doubt. The old adage “When in doubt, don’t” is often a good course to follow.
When faced with a choice, ask: “Will this gladden the heart of God or grieve Him?” A good prayer for you to pray today is: “Lord, I want my heart to be grieved by those things that grieve Your heart and be gladdened with the things that give You joy.”