Paul, a bondservant of Jesus Christ, called to be an apostle, separated to the gospel of God (Romans 1:1)
By introducing himself as a bondservant of Jesus Christ before he referred to himself as an apostle, Paul was reinforcing what Jesus said.
Most assuredly, I say to you, a servant is not greater than his master; nor is he who is sent greater than he who sent him. (John 13:16)
Paul, a called apostle of Jesus Christ.
The words “to be” are inserted, and does not appear in the original text. Paul was, more accurately, a “called apostle.” He was actually a “called-by-Christ-apostle” according to Galatians chapter one. - Wacaster, Studies in Romans, 13
Paul was a “called apostle” by the Lord Himself - just like the other original twelve apostles had been called (Acts 26:15-18).
This meant that Paul had the authority of Jesus Christ behind his mission and message. Paul was not like many of the prophets in Jeremiah’s day who spoke a message of their own devising.
Thus says the LORD of hosts: "Do not listen to the words of the prophets who prophesy to you. They make you worthless; They speak a vision of their own heart, Not from the mouth of the LORD…I have not sent these prophets, yet they ran. I have not spoken to them, yet they prophesied. But if they had stood in My counsel, And had caused My people to hear My words, Then they would have turned them from their evil way And from the evil of their doings.” (Jeremiah 23:16, 21-22)
Paul had willingly devoted himself to the service of the Master, and he had been sent in a very special way by Jesus Christ Himself and had been given special authority by the Lord (1 Corinthians 9:1-18).
However, [Paul] was careful to note that not only had he been “called” but those at Rome had been also (1:6-7). They all had the common call to be Christians. - Pollard, Truth for Today Commentary, 30
What about us?
If God could stoop to pick out the man who had been his greatest enemy and call and equip him to be His greatest messenger, then no one need feel that he may not be reached of God. - Barnhouse, Romans Vol. 1, 15
What stands out to me at the beginning of this letter is the humility of the apostle Paul. Yes - he was an apostle of Jesus Christ and had authority given to him by Christ. But Paul first emphasized that he too was merely a servant of the Lord.
Paul needed the gospel of Christ the same as everyone in Rome - the same as every person today.
No matter what you have done, you can still be reached by the grace and mercy of God. Paul - the called apostle of Jesus Christ - is evidence of that fact, and the message Paul preached (the gospel of Christ) is God’s power to save you today.