How does your church define sin?
In reality, it doesn't matter how any church defines sin. Each of us should be concerned with how God defines sin, and to learn of God's definition, we need to look into God's word -- the Bible.
The most basic definition of sin is found in I John 3:4, "Whoever commits sin also commits lawlessness, and sin is lawlessness." Sin is the breaking of a law, hence in order for sin to exist, there needs first be a law, "for where there is no law there is no transgression" (Romans 4:15) or as Paul later stated, "sin is not imputed when there is no law" (Romans 5:13).
The first record of mankind sinning is when Adam and Eve ate the fruit of the forbidden tree (Genesis 3). It was a sin because they broke God's simple law (Genesis 2:16-17).
In the relationship of law and sin, there is an unfortunate side-effect. God's law defines what is right and what is wrong, but by defining sin it makes people conscience of the possibility to sin. Paul discusses this dilema in Romans 7:7-12. "What shall we say then? Is the law sin? Certainly not! On the contrary, I would not have known sin except through the law. For I would not have known covetousness unless the law had said, "You shall not covet." But sin, taking opportunity by the commandment, produced in me all manner of evil desire. For apart from the law sin was dead. I was alive once without the law, but when the commandment came, sin revived and I died. And the commandment, which was to bring life, I found to bring death. For sin, taking occasion by the commandment, deceived me, and by it killed me. Therefore the law is holy, and the commandment holy and just and good." (Romans 7:7-12). Paul was of the nature not to desire what belonged to another man, but the law made him aware that covetousness existed and with that awareness, Satan had an opening to cause within Paul a desire to covet. The fault does not exist in the law itself, but in the weakness of mankind.
Sin doesn't just exist when there is purposeful violations of the law. We can also sin by simply neglecting to do what is right. "Therefore, to him who knows to do good and does not do it, to him it is sin" (James 4:17). Sin can also exist when we act on the presumption that something is right without ever checking to see God approves of our action. "Keep back your servant also from presumptuous sins; let them not have dominion over me. Then I shall be blameless, and I shall be innocent of great transgression" (Psalm 19:13).
For similar reasons, man sins when he attempts to modify God's law either by adding things into it that God did not put there or by removing things from the law that God did place within it. As God told Israel, "Whatever I command you, be careful to observe it; you shall not add to it nor take away from it" (Deuteronomy 12:32).