No one can serve two masters. Either you will hate the one and love the other, or you will be devoted to the one and despise the other. You cannot serve both God and money. Matthew 6:24
“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 (Matt. 6:21); and he now gives warning, that the hearts of those who are devoted to riches are alienated from the Lord…
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…
What is here said with a special reference to riches, may be properly extended to every other description of vice. As God pronounces everywhere such commendations of sincerity, and hates a double heart (1 Chron. 12:33; Ps. 12:2), all are deceived, who imagine that he will be satisfied with the half of their heart. All, indeed, confess in words, that, where the affection is not entire, there is no true worship of God.”
John Calvin in Commentary on harmony of the evangelist, Matthew, Mark, and Luke, trans. William Pringle (Calvin Translation Society, Edinburgh, 1845) 337-338.