You reap (more than) what you sow.
I know personally that this is true. The two years following my first true consistent tithe (10% of all income), my wife and I made more money and were blessed with more than in any other decade in our lives. I'll spare you the amount because that doesn't matter.
We didn't start giving out of a desire to be rich.
We obeyed out of a heart to acknowledge God,
to please and glorify Him, and to be used by Him.
God knows our heart. All he wants is obedience and trust in His promises. And with that obedience and trust comes blessing. This is what His Word clearly states, and I have personally experienced this Truth, many times over now. But that's me and my family, because we choose to serve the Lord.
Only about 3% of proclaimed "Christians" tithe regularly. Three percent. Imagine if even 25% of proclaimed Christians tithed regularly! What would that amount be, and what would God do with that? Imagine if 50% tithed ... or 100%! Just think about that!
I'm not even talking about more than 10%. I'm just saying the minimum of 10% (where we keep 90%!) to show God that we know it all (100%) came from Him.
And what if we all gained confidence through that first step of trust and obedience and started giving 15% the next year, then 20% the following, then 25% the year after, and so on?
You reap (more than) what you sow. This is a promise and a challenge. Will you trust Him at His word? I did, and I can testify to His faithfulness.
"God set it up this way because he wants you to become like him. God is a giver. The only way that you're ever going to become like him is to learn to be generous. When you're stingy, you're like the devil. When you're generous, you're like God. When you give it away, God multiplies it."
-- Rick Warren