Sin is a word bandied around without a great deal of understanding about what it actually is. It is used in the modern day to refer to anything from a trifling peccadillo, eating the wrong food on a diet or a general sense that something is bad behaviour.
In his book The Sinfulness of Sin, the Puritan Ralph Venning offered this definition:
Sin is the transgression of a law, yea of a good law, yea of God’s law. Sin presupposes that there is a law in being, for where there is no law there is no transgression (Romans 4:15). But where there is sin, there is a law, and a transgression of the law. Whosoever committeth sin transgresseth also the law, for sin is a transgression of the law (1 John 3:4). That this is the sin intended in our text is apparent from Romans 7:7.
Now the law not only forbids the doing of evil, whether by thought, word or deed, but also commands the doing of good. So to omit the good commanded is sin, as well (or ill) as is the doing of the evil that is forbidden. Against the fruit of the Spirit there is no law, but against the works of the flesh (for the antithesis holds) there is law, for they are all against the law, as the Apostle tells us (Galatians 5:19-24). Whatever, then, transgresses the law of God – in whole or in part (James 2:10) – is therefore and therein a sin, whether it break an affirmative or a negative precept i.e. whether it is the omission of good or the commission of evil.
In other words, sin is anything that fails to obey any command of God. It is a failure to do the good that God commands or to commit behaviour that God forbids. This includes the things we think, say and do. Sin is the breaking of any command of God – positive or negative – worked out as either the failure to do good or the active doing of what is evil in thought, word or deed.