I was once asked by a member of our church a very trivial question. It needed a lot of thinking and the leading of the Holy Spirit to answer, and by God's grace I was able to shed some light that day.
The question went something like this: "If I were to make a living from professional lottery betting and give a very significant portion to the poor and to the church, would gambling still then be a sin?"
The Bible says nothing directly against gambling, betting and the lottery, but that's not to say that the Bible is incomplete or that gambling just took a slide. Although the Bible does not give us a picture of how we are not to earn money, it is filled with stories of how we are supposed to earn money.
God reminds us of two truths when it comes to earning money: that it comes solely because of the grace of God, and that the most natural way that God allows us to earn money is to work for it, not win it.
The Bible has nothing against getting rich, that's true, but it does warn us against getting rich too quickly, which is what most people really desire when they gamble or bet.
Proverbs 13:11 tells us that "Wealth gained hastily will dwindle, but whoever gathers little by little will increase it." When we gamble, we put ourselves in a spot where we set ourselves up for destruction. A Biblical example would be the prodigal son. Though he didn't get his wealth through gambling, God wanted to tell us how quick money will often spell quick danger.
There are also those who argue that gambling can be a form of leisure for those who have disposable wealth to squander, but isn't that just wasteful and selfish?
The Bible also speaks of the love of money, which comes easily when we have too much of it right away. I remember the time I started one of my most successful business ventures prior to becoming a full-time minster. I was 18 years old and earning a lot of money. Was I sinning by working hard? No, I wasn't, but the true goal of my work was to honor my wallet, not God. That was sin, and I had to repent.
Luke 16:11 tells us, "If then you have not been faithful in the unrighteous wealth, who will entrust to you the true riches?" Gambling is bad not because we earn money. It can be treacherous because it is often always a given that we try to earn money without the God-ordained purpose of working hard for it and trusting Him to bless us because of our labor.
That's not to say that we work hard and think it's about our ability. At the end of the day, it's about honoring God and doing things His way, trusting that He will make things work out for us.