Question: "Is it a sin to play poker?"
Answer: Poker is often considered a sinful activity due to its close relationship with gambling. The Bible instructs us to keep ourselves from the love of money (1 Timothy 6:10; Hebrews 13:5). The Word of God also teaches us to stay away from attempts to "get rich quick" (Proverbs 13:11; 23:5; Ecclesiastes 5:10). Further, there are far more beneficial things we can be doing with our money than wasting it on gambling (Luke 6:38; 2 Corinthians 9:7).
However, there is nothing essentially sinful about playing with cards. Apart from gambling, playing poker is no different than playing a board game. Poker can be played with chips (or other substitutes) instead of money. If gambling is removed from the "equation," there is nothing wrong with a friendly game of poker. The key is playing in moderation, not allowing it to become an obsession, and avoiding the temptation towards gambling.
An additional consideration regards the principle of not causing a brother to stumble (1 Corinthians 10:31-33). If, by playing any card game with someone who may have a problem with the temptation to gamble, we knowingly cause him to be tempted further, then we are doing harm to that person. As in all things, love for others should be a primary consideration.