No one can serve two masters; for either he will hate the one and love the other, or else he will be loyal to the one and despise the other. You cannot serve God and mammon.

Matthew 6:24 (NKJV)

Christ returns to the former doctrine, the object of which was to withdraw his disciples from covetousness. He had formerly said, that the heart of man is bound and fixed upon its treasure; and he now gives warning, that the hearts of those who are devoted to riches are alienated from the Lord. For the greater part of men are wont to flatter themselves with a deceitful pretense, when they imagine, that it is possible for them to be divided between God and their own lusts.

Christ affirms that it is impossible for any man to obey God, and, at the same time, to obey his own flesh. This was, no doubt, a proverb in common use: No man can serve two masters He takes for granted a truth which had been universally admitted, and applies it to his present subject: where riches hold the dominion of the heart, God has lost his authority. True, it is not impossible that those who are rich shall serve God; but whoever gives himself up as a slave to riches must abandon the service of God: for covetousness makes us the slaves of the devil.

~ John Calvin’s Commentary on Matthew 6:24