"not to be served, but to serve"

"God, Who is the first agent of all things, does not act in such a way that something is attained by His action, but in such a way that something is enriched by His action.  For He is not in potency to the possibility of attaining something; rather, He is in perfect act simply, and as a result He is a source of enrichment.  So, things are not ordered to God as to an end for which something may be attained, but rather so that they may attain Himself from Himself, according to their measure [sed ut ab ipso ipsummet suo modo consequantur], since He is their end."

St. Thomas Aquinas, Summa contra Gentiles III.18.5, as quoted in Reinhard Hütter, in his "Aquinas on the natural desire for the vision of God:  a relecture of Summa contra Gentiles III, c. 25, après Henri de Lubac," The Thomist 73, no. 4 (October 2009):  544 (523-591).