The beloved apostles reasoning for supporting missionaries. Note: John had the heart of Christ like no other. Paul clearly agrees with John's missions philosophy in the last chapter of Romans.
4 I have no greater joy than to hear that my children walk in truth. 5 Beloved, thou doest faithfully whatsoever thou doest to the brethren, and to strangers; 6 Which have borne witness of thy charity before the church: whom if thou bring forward on their journey after a godly sort, thou shalt do well:
7 Because that for his name’s sake they went forth, taking nothing of the Gentiles. 8 We therefore ought to receive such, that we might be fellow helpers to the truth.
Doing "well" is contingent upon supporting the spead of the Gospel throughout the World! If you are not supporting Missions, there is something wrong.