The kingdom of God is the sphere in which God exercises His authority so that He may express His glory for the fulfillment of His purpose. In such a kingdom, what matters is not eating and drinking but righteousness, peace, and joy in the Holy Spirit. Righteousness denotes that which is right and proper. Those who live in the kingdom of God should be right and proper toward others, toward things, and toward God; with them there should be nothing erroneous, improper, crooked, slanted, or biased. This requires that they be strict in dealing with themselves. Peace is the fruit of righteousness (Heb. 12:11 and note). It characterizes the relationship that those who live in the kingdom of God should have with others and with God. If we are righteous, right, and proper toward others, toward things, and toward God, we will have a peaceful relationship with others and with God. Thus, we will have joy in the Holy Spirit and, in particular, before God. In this way we will be filled with joy and with the Holy Spirit (Acts 13:52) and will live out righteousness, peace, and joy in the Holy Spirit, which are the reality of the kingdom of God.
According to the context of this chapter, this verse was written for our receiving of the believers. If we receive the believers according to the apostle's instruction in this chapter, we will be right and proper toward those whom we receive and will have peace with them; thus, we will have joy in the Holy Spirit, proving that we are living in the reality of God's kingdom and are under God's rule. Otherwise, we will not be right or proper toward those whom we do not receive and will not have peace with them; thus, we will not have joy in the Holy Spirit before God, proving that we are not subject to God's authority in the kingdom of God.