These six verses are all tied together by humility—that one should not think of himself more highly than he ought. God has put us each in the body as it pleases Him, so we should not think that we, as, say, the toe are better than the knee because the toe cannot do the knee's job. God thinks of the toe just as highly as He does of the knee, but if He has put us as a toe, why not in faith do the job of a toe because that is what God wants us to be? If He had wanted us to be a knee, He would have put you in the body as a knee, but He made us to be a toe, so be happy as a toe! Do a toe's work in faith!
Paul tells us to think soberly, logically, seriously, that as God has dealt to each a measure of faith, that we in faith can consider our place in the church and deal with it. So, whatever we are to do, do it! Do it with all the gifts and skills that God has given—but do not try to do another's job. It is his job to do diligently, not ours. God put us in the body to do a specific job, our job not his, otherwise He would have given us his job!
If we have been given the job to exhort, then we should exhort. If it is our job to minister and serve others, serve—but do not take another's job to prophesy. Paul is saying, "In lowliness of mind, be content where you are, because obviously God has put you there for a reason. If you do the job that God has given to you, you are fulfilling His will." The church, then, can be united because the members are not competing over each other's responsibilities.