What a reassuring promise! Philippians 4:19 tells us that God will supply our every need, but notice that this verse comes after discussion of the support that the Philippians gave to Paul. (Philippians 4:15-19) We cannot take this verse out of the context of the rest of the Bible; we must look at it in light of other scriptures.
Throughout the Bible we are told to give generously. The ancient wisdom of Proverbs tells us that those who are generous will be blessed, but those who withhold their wealth will come to poverty. (Proverbs 11:24-25, Proverbs 22:9) In Malachi 3:10 we are encouraged to bring the whole tithe into God’s house. This is one circumstance in which God invites us to test Him, and He promises to pour out a blessing upon us from the windows of heaven. Some people believe, or perhaps just use as an excuse, that this was an Old Testament law, and no longer applies to those of us living under the grace of the New Testament. However, this is essentially the same thing that Paul says in Philippians 4:19, God will supply your need according to His riches. Luke echoes this in Luke 6:38: Give and it will be given to you, good measure, pressed down, shaken together and running over. I always think of baking when I read this verse. When you fill a measuring cup with flour, then press it down and shake the cup a bit, it creates room for more. This is the way that God gives; He presses down the blessings, shakes the air bubbles out and keeps on giving past the point where there is room to hold any more. But our giving has to come first.
When Paul thanked the Philippians for giving to him, he emphasized that it was not because he needed more (though it certainly was a blessing to him), but so that they would be acknowledged for their gift. Their generosity was an indication of their hearts toward God, as they were really giving to Him. (Matthew 25:40) God is pleased with such sacrifice (Hebrews 13:16), and He loves a cheerful giver. (II Corinthians 9:6-8) If you give generously, you will be blessed, and both the giver and receiver will experience joy in the gift.