Sin

Sin is not limited to things that we shouldn't do.

James 4:17
Therefore, to one who knows the right thing to do and does not do it, to him it is sin. (KJV)

What aught we do?
1 Thesselonians 5:16-22
16)Rejoice always;
17)pray without ceasing;
18)in everything give thanks; for this is God's will for you in Christ Jesus.
19)Do not quench the Spirit;
20)do not despise prophetic utterances.
21)But examine everything carefully; hold fast to that which is good;
22)abstain from every form of evil.

OK, what else?

Matthew 28:16-20
18)And Jesus came up and spoke to them, saying, "All authority has been given to Me in heaven and on earth.
19)Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit,
20)teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age."

And while you're at it, might as well go for the gold:

Luke 10:27

27)And he answered, "(A)YOU SHALL LOVE THE LORD YOUR GOD WITH ALL YOUR HEART, AND WITH ALL YOUR SOUL, AND WITH ALL YOUR STRENGTH, AND WITH ALL YOUR MIND; AND YOUR NEIGHBOR AS YOURSELF."

Remember, if you know what to do, (and you do) and you don't do it. It's sin.