Luke 16:9

And I tell you (learn from this), make friends for yourselves (for eternity) by means of the wealth of the un-righteousness (that is, use material resources as a way to further the work of God), so that when it runs out, they will welcome you into eternal dwellings.  

As believers, we owe a debt to those who came before us and invested in our salvation by supporting the church.  The work of God is essential, and the spread of the word to the lost is part of our calling.  The next generation of saved Christians is our investment while we are here.