“Paul, a slave of God, and Apostle of Jesus Christ for the faith of the elect of God and the knowledge of Truth which is according to godliness, with the hope of eternal life which was promised by God who never lies from the ancient times,”
Not only is Paul a slave of God, he is an Apostle of Jesus Christ. The term “apostle” was not unique to the Christian world and one who carried such a title was much like what we would call an “envoy” in English today. In principle, an Apostle is one who is commissioned by a person of authority (like a king or governor) to represent him in another locale. The Apostle carried with him the full authority of the one sending him and had the ability to make treaties and agreements with the full authority and backing of his king.
Thus, when the Apostles were sent out, they were given the authority to cast out demons, heal, and speak with the authority of Christ himself (Matthew 10:1; Mark 3:14-15)…hence the Apostolic authorship of the Scriptures. The real debate within Christian circles is two-fold: how many Apostles were there and did the office of Apostle continue into the life of the church?
The first question is really the trickier of the two. The original 12 followers of Jesus are clearly called “Apostles” as is Paul. Clearly the original 12 had a sense that 12 was the proper number, hence when Judas fell away, Matthias was chosen in his place (Acts 1:21-26). Yet, while many consider him an Apostle, he is never explicitly called such in the Scriptures. Further, Luke seems to imply that Barnabas was an Apostle (Acts 14:14). Further, many of the early church fathers attributed Apostleship to others in the life of the early church.
The best answer seems to be to ask the question, “whose authority do they carry?” Is it the authority of Christ or is it the authority of the church? When the early church looked at the Apostolic authority of Christ as the basis for establishing the Canon (which books included in the Bible?), they were not looking to Barnabas nor were they looking at writings of some of the other early church fathers that are sometimes referred to as Apostles. They looked to the original 11 (12 minus Judas) and to Paul. These seem to be the divinely inspired 12 who are Apostles who speak with the authority of Christ. The others could arguably be considered apostles of the church, carrying with them the church’s authority, but no more.
If that then is the case, why not still call pastors “Apostles”? The answer is simple: the Bible doesn’t, and thus we should not presume upon ourselves a title that the Bible does not give to us. And that leads us to the second question…did the office continue? Some Pentecostals and certainly Rome would argue for the continuation of the office and the authority that comes with that office, yet they operate outside of both the Bible and church history by doing so. By the time we see the Council of Jerusalem taking place in Acts 15, we find that the Presbyters of the church (the Elders) are acting as the normative ruling body for the church alongside of the Apostles. Hence there is a transition of leadership that took place where the normative office of Elder replaces the temporary and special office of Apostle in church leadership.
Thus, Paul, as he opens his letter is humbly asserting his authority to guide Titus. Does this mean that Paul is trying to strong-arm Titus into making sure Titus does things properly? Of course not, such is not the character of Paul nor is it characteristic of the relationship between these men. A better answer, given that Titus is Paul’s emissary on Crete, is that Paul is lending his weight to what it is that Titus is seeking to accomplish, that way those stubborn folks in the church there won’t challenge Titus’ authority to establish Elders in the church. And though pastors today are not operating under the direct commission of Paul, we too can appeal to these letters and say, “the things we do we do not do on our own authority…it is on the authority of the Apostles that God established to found his church.