Jesus has been presenting choices; there is the choice between two treasures, the choice between two eyes, and now a choice between two masters. You can’t serve both, and each moment you are serving one or the other.
The world gives what it thinks is noble advice: master your money, don’t let it master you. However, this is where people get fooled. Trying to be the master at all is the problem in the first place. Jesus rightly tells us that this is not an option; we are the servants, and either God or money will be the master.
For the Christian there is not supposed to be any question, we are bought with blood money (1 Corinthians 6:20 / 1 Peter 1:18-19). It isn’t “get control of your money before it gets control of you”, it’s “if God isn’t in control of your money, He isn’t in control of you, and then money is your master, not you”.
Whatever you place your trust in is your God. We are supposed to be slaves of God who are stewards of what He entrusts to us, instead of slaves to ourselves and what we entrust ourselves to. People think this isn’t subtle but it is, and the slave of money will obey money while pretending to obey God. We must plan to build spiritually by purposing to serve God financially, otherwise, we will fall prey to the wrong master.
Will you be mastered by God, or mastered by greed? There is no middle way.