One sows to the Spirit (one does good by preacheing to build faith) => One reaps from the Spirit (one receives life eternal – something good, i.e. here on earth [salvation] or there in heaven [the great reward])
IN DUE SEASON WHENEVER THAT MAY B IN GOD’S TIME
Galatians 6:6-10 ESV
Let the one who is taught the word share all good things with the one who teaches.  Do not be deceived: God is not mocked, for whatever one sows, that will he also reap.  For the one who sows to his own flesh will from the flesh reap corruption, but the one who sows to the Spirit will from the Spirit reap eternal life.  And let us not grow weary of doing good, for in due season we will reap, if we do not give up.  So then, as we have opportunity, let us do good to everyone, and especially to those who are of the household of faith.