Jesus continues the theme of loyalty to God: one cannot serve two masters. Service here is to be a slave, not be employed. It is possible to be employed by two masters, but not to be a slave. Slavery requires complete devotion, a slave was to be 100% disposable to his master. One cannot give 100% to two different persons, so a slave cannot serve two masters. By way of analogy, imagine a baseball player who is trading to a new team, but wants to continue serving the old team. If he were to do things to help the old team, the new team would be furious. Imagine a player trying to wear his old uniform with his new team.
The reference to loving one, hating the other probably implies that he serves both, but is not devoted to both. One of the services is hypocritical and shallow, the other is genuine, but not one hundred percent.
The ESV and NIV translate μαμωνᾶς as money. The KJV transliterated the Aramaic word (מָמוֹנָא). BDAG suggests this was a Canaanite loanword originally meaning “food, maintenance, provisions” (מון). In Luke 16:9, 11 the word simply means money. Mammon is not the name of the “demon of materialism.” Some Aramaic Jewish writings translate Proverbs 3:9 as “honor God with your Mammon” and Deuteronomy 6:4 include Mammon in the list of things which can be used to love God.
Nevertheless, Jesus is personifying money and possessions as the opposite of God, one can either choose to seek first money or seek the kingdom of God.
Jesus reflects the general teaching of wisdom literature on the proper use of wealth (Prov 3:9), although without the promise of material blessing in the future. As with the first saying, there is a hard edge of coming persecution in Jesus’s claim one can only serve one God. As his disciples are persecuted, they will have to choose between their possessions and God. Under threat from Rome, disciples can either choose the nice home and comfortable life, or God and his kingdom.
This is an important (and convicting) application of Jesus’s words for modern America. If there was a government sponsored attack on the church that forced a decision to following Jesus and give up our wealth as a church, what might we choose? If standing firm on an issue cost a church their tax-free status, would they continue to stand firm?
Jesus never says wealth is evil nor does he demand his followers all join him in a voluntary vow of poverty. But he does call his followers to live a life of simplicity, to hold their possessions lightly, and to dedicate themselves to pursuits which result in eternal profits.