What NT evidence is there for the idea that the apostle Paul's introductions to his letters were to authenticate his position as an apostle --- whether he was concerned to establish his credentials at the outset, or whether he was concerned to point to how he (and others) could be in a personal relationship with a living Jesus Christ.
A -- Paul’s Relationship with Christ and His Apostleship - Generally Considered
I would say immediately that it is impossible to detach some one like Paul from a vocation like the one to which Paul was called. Indeed, from the very outset of Paul’s Christian experience, his personal meeting with Christ and his calling to preach the gospel were intertwined and inseparable, Acts 9:15.
Thus when Paul introduces himself in his epistles he does so in two ways and these ways are coincidental : his personal calling to Jesus Christ, Galatians 1:15-16, as a penitent sinner baptised into the Body of Christ ; and as an apostle, called by Jesus Christ for a specific function and a specific task, 1 Corinthians 1:1.
I believe one cannot separate the one from the other.
And, that said, it is instructive to see that both are dependent upon one another. For someone in Paul’s position, who says ‘To me, to live is Christ’, Philippians 1:21, his own relationship to Christ is dependent on his fulfilment of his calling. ‘Woe is unto me if I preach not the gospel’, he says, 1 Corinthians 9:16. He was called for a purpose. He was singled out for a task.
Massive - incalculable - is the eternal benefit to he, himself, if he is obedient to that calling, 2 Timothy 4:8. But woe be to him, if he does not responsibly fulfil it.
Nor does he depend on his ministry and his apostleship for salvation, irrespective of his relationship to Christ and his own maintaining of a holy walk. ‘I bring my body under subjection, lest, having preached to others, I myself become a castaway’, 1 Corinthians 9:27.
He recognises that it would be possible to minister the gospel, yet, himself, to be lost.
So the man, Paul, and the minister, Paul, are one and the same.
And the believer, Paul, and the apostle, Paul, are one and the same.
I cannot see that Paul, in his epistles, written or dictated by himself, and his conduct, recorded by Luke in Acts, ‘compartmentalises’ himself ; at one time being one thing and at one time being another. Or at one time ‘wearing one hat’ and at one time ‘wearing another’. He just does not do it, as far as I can see.
Though he does, in one place, assert his rights as a Roman citizen, but that is a judicial matter, and a matter of legal right, rather than a personal or spiritual issue. It was a right granted to him by men, which, at a certain time, he chose to make use of, expediently, Acts 21:39.
It is also conspicuous that Paul’s ministry is to minister what he himself has and what he himself experiences. He does not minister what is not his. He does not ever quote anyone else, other than scripture and a single poetic expression regarding Cretans, uttered by a fellow-Cretan : a shrewd move when criticising persons ethnically, to quote the criticism in the mouth of one of their own, Titus 1:12.
Aristotle, Plato and other like persons are utterly absent from Paul’s writings. Scripture is his only reference, and that not as interpreted by others, but solely the reference itself. Paul never ‘steals his words from his neighbour’, Jeremiah 23:30. It all comes from his own meditations, his own knowledge of scripture and from his own personal experience.
He ministers from out of his own union with Christ.
‘Be ye followers of me,’ he says,’ as I follow Christ,’ 1 Corinthians 11:1.
He seeks not followers to himself. He labours not to form an entourage, though some voluntarily joined with him and accompanied him and laboured with him. But sometimes not : sometimes he finds himself alone : all have forsaken, but one or two, 2 Timothy 4:10. ‘All seek their own things and not the things which be Jesus Christ’s,‘ Philippians 2:21.
But, whether in want, or whether in prosperity, Paul learned to be content, Philippians 4:12. Christ was enough : to live unto and to die unto, Romans 14:8. To befriend in this world and to be with, for ever : Paul’s only dilemma being whether to depart and be with his Lord, or to remain and help others, Philippians 1:23.
B -- Paul’s Introduction in Galatians, Specifically Examined
Paul begins the first verses of his epistle to the churches of Galatia, naming himself, Paul, and immediately titling himself, apostle. And the original says exactly that - Paul apostle. In Greek, to say ‘the apostle‘ would denote that there was only one. And the absence of the article does not actually give warrant to add our English ‘indefinite article’ (neither Hebrew nor Greek need one nor want one).
And immediately, he denies, with his third word, receiving his apostleship from any human source whatsoever, either in origin, ‘of men’, or in agency ‘by man’. Paul expresses his apostleship to be by Jesus Christ, directly and without any intermediate.
No institution. No governing body. No approval. No appointment, No exams. No graduation.
All of it was personal, between himself and his Lord.
None else stood in between.
This is what gave Paul his authority. This is that alone which prompted his ministry, his apostleship and his epistle - that all was directly from Jesus Christ and through himself, personally.
What he was, he was because Jesus Christ had chosen him, ‘a chosen vessel’ Acts 9:15.
Nor, after that choice being made known, did he confer with any other, but he went into Arabia and then to Damascus. Three years later he stayed with Peter for a fortnight, but saw none other. Fourteen years later he went to Jerusalem, due to controversy.
But he gave place to none, for those who seemed to be somewhat, in conference added nothing to him that he did not already have.
And where had he received it ? He received it from the same source that Peter had received it : from Jesus Christ, personally.
This - and nothing else - is apostleship.
These are the Ministers, sent of Jesus Christ.
And this - thoroughly, absolutely and continuously - is personal.
This is what gives Paul such authority that his epistles are read, and studied, world-wide, to this day.