This question is a common one, and it’s usually thrown out in a particular category of things. Is drinking sinful? What about smoking, swearing, and speeding? Those things all tend to fall into the same category, at least when the question of whether or not they are sinful is raised.
Up until recently, these are the sorts of question I asked all the time. Then I heard an episode of John Piper’s short form podcast “Ask Pastor John” where he explained that those are the exact wrong types of questions to ask. Those questions are asked when we’re trying to live as minimal Christians. What I mean is that the temptation is to try and find out just how close we can get to the fire without getting burned. As Christians, we don’t want to try and get by by the skin of our teeth (where did that phrase come from?). We should be asking the types of question that would lead toward maximal Christian living.
Is gambling going to allow me to more accurately reflect the glory of God? Will gambling bring me closer to Christ? These are more helpful question to ask when regarding this topic, and ones to which the answer becomes much more obvious. I cannot think of a way in which the answers to those question would be “yes”. However, I think we can push things further.
I’ve often heard gambling placed in the category of entertainment, and so it can be thought of as “neutral”. like you might think of basketball. I’m again inclined to agree with Piper on this, as he points to the motivations of gambling being quite different from other forms of entertainment. We are to be good stewards of all God has given us, including our finances, and it seems impossible to me to be able to argue that taking some money to gamble in hopes of getting rich is a form of good stewardship.
You can give further reasons which get farther off the point, but I think those two are important enough that we can safely say that gambling is sinful, and should not be done by Christians.