Unclean
Pig sties, outhouses, your roommate's closet - these things might come to mind when you think of unclean. In the Old Testament the list was much longer. The things that made people unclean included association with death, touching certain things (see Touch), eating certain animals, infectious diseases of many kinds, and of all things, mildew! God's detailed instructions on how the Israelites should keep themselves physically pure underscored his holiness and incompatibility with anything impure. Bug Jesus made it clear in the New Testament that these rules had merely illustrated a concept that would be more clearly delineated at a later time.

We are unclean because of what is in our hearts. We are born with sin; the inclination to rebel is present before we even have the capacity to act on it. Sinful actions are sinful because they result from a rebellious heart. A seemingly good action, carried out for selfish purposes, is a sin.

In the Old Testament, God identifies many things that will make the Israelites unclean.
Leviticus 11:1--15:33
The state of our hearts makes us more impure than what we do externally.
Matthew 5:27-28
Mark 7:1-23
We should guard against the impurity that comes from sinful habits that are rooted in sinful desires.
Ephesians 5:3-5
Colossians 3:5-10
1 Thessalonians 4:3-8
We are cleansed and made acceptable to God through Jesus Christ.
Hebrews 9:9-14
2 Peter 1:9