Therefore one who follows God, the Father of the Lord Jesus Christ, must have one focus and have his treasure in Him alone.
These verses makes it clear that there are two masters in this life. God is one and should be the only one in your life, but there is another. Jesus called it mammon (or money) while others call it the world or anything other than God.
In reality “a man may do some service to two masters, but he can devote himself to the service of no more than one. God requires the whole heart, and will not share it with the world. When two masters oppose each other, no man can serve both. He who holds to the world and loves it, must despise God; he who loves God, must give up the friendship of the world.” Matthew Henry Concise Bible Commentary.
“The two masters Jesus speaks of are God and Money. "Money" here means any kind of worldly treasure: land, house, animals, family, education, job, authority, fame. In place of the word "money" we could substitute the word "self."
Every person must choose which master he will serve. In the same way, everyone must choose which kingdom he will live in: the kingdom of God or the kingdom of darkness. No one can be a citizen of both kingdoms at the same time.” Interpreter's Bible, The - Exposition - The Interpreter's Bible – Volume 7.
Whichever master we serve we are a slave to them. Paul said that he was a slave to the Lord Jesus Christ. 1 Corinthians 7:22,