“Each one must do just as he has purposed in his heart, not grudgingly or under compulsion, for God loves a cheerful giver.”
-2 Corinthians 9:7 New American Standard Bible (NASB)
For those who do not feel the call to go out in the mission fields they can still play an important part by supporting those who go financially.
God calls each of us to fill the gap in order to spread His Gospel to all parts of the world. Giving so that God’s servants can take His message throughout the world, like tithing, is an outward material expression of a deeper spiritual commitment and is an indication of a willing and obedient heart. Regardless of the work to which we are called, there is hardly a Christian who cannot give; and when that giving is done in love, it exemplifies the greatest sacrifice ever made for mankind: the death of Jesus on the cross.