The one title which is for all believers is 'Ambassadors of Christ'. In Ancient Rome this meant:
Roman provinces were divided into two types. One was under the direct control of the ruling Senate and the other under the direct control of the Emperor. The second group were turbulent and had to have soldiers stationed in the area to keep the peace. The man who administered these provinces on behalf of the Emperor was called the Ambassador and this is how Paul regarded himself: commissioned by Jesus Christ for the work of the Church. The Roman Ambassador arranged the terms of peace, determined the physical boundaries, drew up a constitution and brought new members into the Roman Empire. Paul saw himself and potentially all Christians as having a similar vocation.
Ambassadors lived in a foreign land; Christians are called to live in the world, although he or she is a citizen of heaven. Being a Christian means recognising that you are different then others.
The Ambassador speaks for his own country in that place. Christians are called to speak for Christ, often bringing the message of God into different situations which are faced in that country.
The honour of the country he is representing is in the hands of the Ambassador. The honour of Christ and of the Church are in your hands: by your words and by your actions, you can draw others closer to Jesus or send them running in the opposite direction!
Paul reminds his readers of the actions of God to make it possible to become His children. God has sacrificed Himself to bring men and women into His Kingdom. Paul pleads with the believers in Corinth not to have accepted the offer of the grace of God to no purpose at all, because they end up walking away: in that instance, Christ is crucified all over again and the heart of God is broken.