James 4:17, "Therefore, to him who knows to do good and does not do it, to him it is sin."

This refers to the sin of omission. Typically we speak of sins that have been done or committed, but in this case we are referring to things left undone, omitted. Because life is short and we do not know the number of our days, we should be ever mindful of our duty to Christ. Christians should not neglect the commands of the New Testament. To do so would not be giving Christ the glory due Him, nor would it help guide others to His service.

"Mat 5:15 Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house.
Mat 5:16 Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven." We must let our light shine not for our benefit but for the glory of God.

"Rom 12:1 I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service.
Rom 12:2 And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God." Here we are told that the Christian behavior is a lifestyle, not a random act of goodness. Also, our imitation of Christ proves the will of God which is called good, acceptable and perfect.

James has also said that every good thing comes from God, James 1:17. In turn Paul writes,"Rom 12:9 Let love be without dissimulation. Abhor that which is evil; cleave to that which is good." And also this, "Rom 12:21 Be not overcome of evil, but overcome evil with good."

"Gal 6:10 As we have therefore opportunity, let us do good unto all men, especially unto them who are of the household of faith." Here we can see another similarity to what is written by James, namely, working while we have the opportunity.

By this scripture we can surely know what is good, "2Ti 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
2Ti 3:17 That the man of God may be perfect, throughly furnished unto all good works." We have everything we need to be pleasing to God and acceptable in His sight.

If we do not choose what is "good," then we have already chosen that which is evil. James 4:17 "Therefore, to him who knows to do good and does not do it, to him it is sin."