It is interesting how Paul labels himself at the beginning of each letter:
Servant/Slave and Apostle: Romans, Titus
Apostle: 1 Corinthians, 2 Corinthians, Galatians, Ephesians, Colossians, 1 Timothy, 2 Timothy
None: 1 Thessalonians, 2 Thessalonians
We could look at those in more depth, for example in Romans he is a servant of Christ Jesus called to be an apostle and set apart for the gospel of God, whereas in Titus he is a servant of God and an apostle of Jesus Christ to further the faith of God’s elect. Paul clearly used God and Jesus interchangeably with reference to his apostleship and the gospel. He will also use some of the alternate titles for himself in the body of his letters. e.g. in Ephesians 4:1 he calls himself a prisoner for the Lord.
Here is Ephesians, he calls himself an apostle of Christ Jesus by the will of God. This matches with the content because he soon explains how Christians have been chosen by the will of God. Note how Holy Spirit is included in the equation. His presence is in contrast to the previous evil spirits that influenced them. In chapter two he turns to the subject of Gentiles and Jews in the body of Christ and affirms that they are joined by the one Spirit.
At the beginning of chapter three Paul labels himself the prisoner of Christ Jesus for the sake of “you Gentiles.” Clearly he is referring to his status and is speaking to Gentile Christians. He then celebrates the great deed and mystery of God in admitting the Gentiles into the people of God and forming the church. In this way “This manifold wisdom of God should be made known to the rulers and authorities in the heavenly realms” (3:10). This is quite in contrast to their former ways when they followed the ways of the world and the ruler of the kingdom of the air (2:2). The chapter concludes with a prayer in which he affirms that all families are from God and celebrates God’s work among them.