It ought to be fairly easy to see that one cannot serve two masters, as Jesus states in Matthew 6:24. It ought to be, but it isn’t because the human worship-system contain an ever-ready capacity to steer off course. Parents find it hard to make good decisions on how much time to spend with children; spouses choose work over family; families have a hard time figuring out to go to church or not; employers find it difficult to be as generous as they ought and workers find it hard to devote themselves to the job they’re supposedly committed to. Many have a hard time deciding how fast to drive — I could go on. Humans, wired as they are to love God and others, find it hard to do just that.
And the special “master” Jesus particularly has in mind is the Master Mammon. Jesus: “You cannot serve both God and Mammon.” Humans: “Let me show you how.”
“Mammon” is the English transliteration of the Greek transliteration of the Aramaic term “mammona” which means “money” or “possessions.” Which means we are back to 6:19-21: Jesus is concerned here with the trust system and the money/possession system. Trusting God is where it is at; trusting Master Mammon is idolatry. The “bad” eye serves Master Mammon; the “good” eye serves God.
As Dale Allison says, Master Mammon invites us to accumulate; God invites us to give away Mammon. It is the hoarding or the giving that one’s relationship to Mammon is seen. God is either first or he is gone. There are no other alternatives — but “God first” cannot be reduced to personal relationship. For Jesus, “God first” or (to be USA) “In God we trust” expresses itself in giving mammon away. Idolatry is mammonaltry.