Paul reminds the Romans of three core features of the kingdom of God. The first is righteousness. One verse that comes to mind in this regard is one in which Jesus linked the kingdom and righteousness: ‘For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven’ (Matt. 5:20). The righteousness of the scribes and Pharisees was taken up with external conformity to burdensome man-made rules. Christians should not focus on such things. Instead they want a righteousness that flows from the heart of God into their hearts.
Some people suggest that what we have here is a description of the Trinity at work. In this interpretation, the righteousness is that of Christ, the peace is that of the Father, and the joy is that of the Holy Spirit. Others say that what we have here is a description of the order of salvation: we believe in Jesus and receive his righteousness, which leads to peace with God, which leads to joy by the Holy Spirit. They could support this suggestion by turning to Romans 5:1-2: ‘Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ. Through him we have also obtained access by faith into this grace in which we stand, and we rejoice in hope of the glory of God.’ A third suggestion is that righteousness relates to God, peace relates to others, and joy to ourselves.
Of course, those ideas are true, but that does not mean that they are the truth that is mentioned here in Romans 14:17. In the next verse, Paul writes that those three features are the ways by which believers serve Christ and live commendable lives before others. So it looks to me that the three features are practical expressions of brotherly love.
Righteousness is doing what is right in the situation because we want the other believer to grow in grace and not be hindered by us – we could call it the activity that should mark all Christians. Peace is the environment in which all Christians should live, and joy is the experience that all Christians should have together. Such a spiritual state is normal Christianity, and it rises from the desire to serve Jesus in the best way possible.
Paul mentions that we should pursue peace and mutual growth. At the very least, we should be chasing after them as continual priorities in our spiritual experience. This is what kingdom living is like. Surely we want to be good citizens of the heavenly kingdom as we live with one another and as we witness to those still outside of it.