The best way to know whether or not something is sinful is to know the Bible. You need to be immersed in the Word of God. That means you need to spend hours carefully studying the Bible, memorizing verses, and meditating upon them. The psalmist said, “Your word I have hidden in my heart, that I might not sin against you” (Psalm 119:11). If we do that, the Word will take care of the questionable situations that arise. To know what the Bible says is the best way of knowing what is right and what is wrong.
There is another helpful guide. The apostle Paul said, “Let the peace (soul harmony which comes) from Christ rule (act as umpire continually) in your hearts” (Colossians 3:15, AMPLIFIED). An umpire announces, “You are safe,” or “You are out.” When that peace inside your heart disappears, you know “you are out.” Your heart is saying, “This course of conduct is wrong.” When you begin to feel troubled within yourself, you know that what you are doing is ill-advised, improper, or sinful.
Paul made it even clearer when he said, “Whatever is not from faith is sin” (Romans 14:23). This principle covers gray areas of conduct. To the person who lives in France, a glass of wine with dinner may be perfectly in order. That may be his lifestyle. Besides, the water may be impure and the wine helps his digestion, so there is no spiritual problem (see I Timothy 5:23). In America, where there is a high incidence of alcoholism and where many Christians have taken a strong stance against drinking any alcoholic beverage, a glass of wine might cause the downfall of someone for whom Christ died (see I Corinthians 8:8-13). When the Bible does not speak explicitly about something, whether it is sin depends, in many instances, on culture, on the person’s status, and on the level of maturity in the Lord. In areas where there are no definite rules as to what is sinful, it is a question of faith. If you feel that something is wrong, if you have a question in your mind about it, and you cannot do it freely without regret or pangs of conscience, then do not do it. To you it is sinful.