“No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.”
No one can serve two masters, for either he will hate the one and love the other, or he will be devoted to the one and despise the other. You cannot serve God and money. The impossibility of dual service: two masters make incompatible demands, and the servant who tries to satisfy both will inevitably favor one over the other. The word is mammon — Aramaic for wealth or property, treated here as a personified rival to God. Luke 16:13 records the same teaching with the added context of the Parable of the Dishonest Manager. The either/or is not between wealth and poverty but between wealth as master and God as master: wealth as servant is fine; wealth as master is the rival deity that displaces God. 1 Timothy 6:10 says the love of money is a root of all kinds of evils.
Church Fathers on Matthew 6:24
The Lord had said above, that he that has a spiritual mind is able to keep his body free from sin; and that he who has not, is not able. Of this He here gives the reason, saying, 'No man can serve two masters.'
Otherwise; it had been declared above, that good things become evil, when done with a worldly purpose. It might therefore have been said by some one, I will do good works from worldly and heavenly motives at once. Against this the Lord says, 'No man can serve two masters.'
Or otherwise; in what had gone before He had restrained the tyranny of avarice by many and weighty motives, but He now adds yet more. Riches do not only harm us in that they arm robbers against us, and that they cloud our understanding, but they moreover turn us away from God’s service.