In Chapter 5, Jesus laid out the battle plan for His new Kingdom - that His followers will embody a new kind of righteousness that goes beyond mere rule following and is satisfied with nothing less than perfection. By doing that, these disciples will be “lights to the world” whose transformed lives will draw all people to Him.
In Chapter 6, the Master urges His disciples to not only do the right thing - namely, to strive to follow God’s example of holiness - but to do so for the right reasons. It is not enough that His followers “look” good on the outside, Jesus expects that we will experience real transformation that changes us from the inside out.
Jesus begins this section by warning His followers against a common pitfall they might encounter along the way to victory - self righteousness. As we pursue holiness, our temptation will be to do so, not in order to please God, but to draw attention and glory to ourselves. As we see throughout the gospels, this is the temptation that destroyed the witness of the Pharisees. While they put on a good show of being godly people, they really were just in it for themselves. In contrast, Jesus commands us to give no thought to the opinion of others, but instead do our “acts of righteousness” for an audience of one - our Father in Heaven. Whether we give to the poor, pray, fast or do anything good, we should actively avoid drawing focus toward ourselves.
The Lord doesn’t end there. He continues, instructing us that we can only serve one master - or, to put it bluntly, we can only have one God in our life. Worldly possessions, the love of money, the approval of others, worries about the future - all of these are potential idols that draw us away from worshiping our true Lord. Jesus’ message is clear - He is not merely concerned with our actions, nor is He pleased with mere lip service. Unlike the so-called gods of the pagan Greeks and Romans, He is not content with superficial sacrifice. He refuses to be a hobby or side gig. He will not be satisfied with anything less than our full, complete and sincere dedication to His cause.
Note that none of this is phrased as an option or a mere ideal to which we should aspire. The Lord doesn’t say that we shouldn’t have two masters, or that He would prefer we have only one; Jesus plainly and emphatically tells us that to serve Him is to serve Him only. He doesn’t not offer any other terms - we can choose to make Him our Lord, with all that entails, or we can follow another. Those who try to have it both ways, to serve Him and their own interests, will realize in the end that they have failed both.