It's isn't because they lack faith,it's because they lack understanding.
Some are taught that we have to give because it's owned to God. Some just give because it's the what they were taught to do, with no idea or reason why they are doing it. Some give thinking that God is like a slot machine, and that you put money in, and riches come out. There are many reason why believer don't see those verses come to pass in their lives. But this was something I needed to change.
But what it all comes down to the heart. In the O.T, or motivation for giving was that you had to (the famous verse in Malachi), or you'd be curse with a curse. Our motivation today is out of love. We give because we want to be a blessing. We give because we want to see the kingdom of God advance. We give so that others can reach the world. If we give for selfish reason, we are giving with the wrong motives, and therefore will not have it "given back unto you." Again, another area I really had to grow in.
There is a lot more to this, but I'm trying to remember and cover as many points as I can.
Someone posted this, but I want to add more to it,
Ephesians 4:28
"Let him that stole steal no more: but rather let him labour, working with his hands the thing which is good, that he may...
have... to...
give... to him that
needeth."
And another verse that goes great with this is, 2 Corinthians 9:6-13,
The point is this: whoever sows sparingly will also reap sparingly, and whoever sows bountifully will also reap bountifully. 7 Each one must give as he has decided in his heart, not reluctantly or under compulsion, for God loves a cheerful giver. 8 And God is able to make all grace abound to you, so that having all sufficiency in all things at all times, you may abound in every good work. 9 As it is written,
“He has distributed freely, he has given to the poor;
his righteousness endures forever.”
10
He who supplies seed to the sower and bread for food will supply and multiply your seed for sowing and increase the harvest of your righteousness. 11 You will be enriched in every way to be generous in every way, which through us will produce thanksgiving to God. 12 For the ministry of this service is not only supplying the needs of the saints but is also overflowing in many thanksgivings to God. 13 By their approval of this service, they will glorify God because of your submission that comes from your confession of the gospel of Christ, and the generosity of your contribution for them and for all others.
My personal motivation to give is because I want to Bless and sow seed into the ministry that has taught me so much, and in order for them to keep growing and in order for them to reach more people. I am a cheerful giver, I love to give because I know what a blessing it is. And I want to be able to grow in my giving, and be able to help out more ministries, and so I stand in the promises of these verses. I know that when I give (sow seeds) I will receive a harvest, and that harvest will enable me to give more, therefore blessing more people. Also my trust is not in my finances, my trust is in the Lord to be my provider. But I see this promise work in my life all of the time. So for me, it works, and if it isn't supposed to, then don't wake me up. I refuse to be a "poor" Christian. Not because I want to sit on my butt and do nothing all day, but so that I can help reach the world. But in order to do that it takes money. There no way around it. We live in an economical world, and therefore we need finances to get things done.
God Bless