For Christians, the Bible does not mention gambling as a sin. I believe the argument against would be the Bible verse (Matt. 6:24) that says “You cannot serve both God and money”. Christianity has many doctrines and some may believe gambling is a sin, some may not and others may be indifferent about the issue. Islam on the other hand has very strong anti gambling views. For atheists, I think their thoughts on gambling is a matter of morality than faith.