For if he answers, as one who is unlearned and ignorant of philosophy, that the injury sustained is one which regards things that are outside of us, it will be plainly manifest that he knows nothing of that famous saying of Socrates, “Anytus and Melitus may kill me, but they cannot injure me; for it is impossible that the better should ever be injured by the worse.” But if by injury he means a wicked impulse or an evil habit, it is plain that no injury of this kind would befall the wise, by one man serving two wise men in different places. If this sense does not suit his purpose, it is evident that his endeavours are vain to weaken the authority of the passage, “No man can serve two masters;” for these words can be perfectly true only when they refer to the service which we render to the Most High through His Son, who leads us to God. And we will not serve God as though He stood in need of our service, or as though He would be made unhappy if we ceased to serve Him; but we do it because we are ourselves benefited by the service of God, and because we are freed from griefs and troubles by serving the Most High God through His only-begotten Son, the Word and Wisdom.
Source: Against Celsus (New Advent)