First, you need to understand the definition of "sin." Sin is an English word and is terminology associated with archery. It is a word that references the distance between the landing of an arrow to the bullseye. In Christian terms, it basically means "to miss the mark". So, anything that is said or done that is not in the general or perfect will of God is sin. Sin is not merely something evil, but something that is not approved by the Holy Spirit at any given time. It could be that the Holy Spirit telling you to read the Word, but you instead go out and wash your car.
Regarding faith, Romans 10:17 says, "So thenfaith cometh by hearing, and hearing by the word of God."
You can't exercise faith unless you can hear from God. This may come in several different ways- hearing that still small voice of the Holy Spirit, your conscience, intuition, and even knowing God's character and how He feels about something.
I mean, if you gotta go potty, you don't have to ask God for permission. There are practical applications of faith and knowing what you're allowed to do on your own free will without violating spiritual laws.
However, if you're not sure about whether something is sin or not, and you do it anyway, then that's sin because you didn't have faith.
I'll give you a personal example- My former job once required me to survey some government land that could only be viewed by standing on ancient temple grounds that was thick with a spiritual/demonic presence. I knew there would be consequences, unless the Lord granted me permission. It was only my second day on the job, and I didn't want to upset my employer. So, I stood there and asked the Lord. After about 30 seconds, I felt the Holy Spirit say "go". So I said a short prayer, stepped out in faith and walked across this ancient temple for a few minutes and stood upon the rocks that overlooked the land I was surveying. I literally felt the rocks vibrate under my feet and knew I was on enemy territory. However, I also felt the Holy Spirit's protection and have not had any sort of repercussion from that incident. No matter how powerful the enemy might be, If the Lord bade me, then nothing was going to stop me because I walked in faith.