The question of inspiration is, of course, central to understanding Paul. Unitarians have always sat warily by the Apostle but James Freeman Clarke loved him, partly because of Clarke's view of the source of Paul's inspiration...
"WE come now to consider the Inspiration of Paul. This inquiry is necessary before we can properly examine his writings. If, for example, we believe that he was so inspired as to be incapable of error, we must accept all he says, as from God,— even when he seems to contradict the fundamental teachings of Jesus, the dictates of sound reason, or even his own teachings in other places. Our freedom of inquiry being thus hampered, we lose our interest in the investigation. But if we regard his inspiration as an influence which led him up to the loftiest truth, but which did not destroy the freedom of his mind, nor obliterate his past opinions, we shall find great interest in seeing how these new and living convictions gradually emancipated him from his old prejudices, and how, according to the promise of Jesus, he was "guided into all truth." What then do we mean by inspiration ?...
The root and essence of this inspiration was the same in all the disciples and apostles. It was the idea of Christ, formed in their souls. This was the common universal inspiration of all Christians.
The influence of the Holy Spirit is not a mystical influence. It is mysterious only as Nature is mysterious ; it is mysterious as the human soul is mysterious, as the life of plants is mysterious. The source of all life is a mystery; but as soon as life begins, it comes under law, and becomes part of a great order. Mystery therefore is a part of nature, and to believe in mystery does not interfere with practice, with prudence, with good sense, with outward usefulness. But mysticism does interfere with all these. Mysticism interferes with the conduct of the understanding. It despises logic; it antagonizes the reflective intellect. It lives by intuition alone, never correcting its intuitions by observation and reflection; and thus is morbid, because leaving important faculties unused.
Now, when we open the Book of Acts we shall see that the spiritualism of the Apostles was not mysticism. It did not take them away from life, but carried them into life. They were no visionaries nor dreamers; they were neither monks nor hermits. They were the most practical men then living in the Roman empire; for they had the greatest work to do, saw most clearly what it was and how it was to be done, and were doing it with their whole might. Nor did they undervalue the reason. While living in the spirit and walking in the spirit, they were always ready to give an account of their faith, to defend it by facts and arguments. He was surely no mystic who defended himself before Felix and Herod at Jerusalem, who argued with Stoics and Epicureans at Athens, and pleaded before Caesar at Rome. In apostolic times, when the whole life of the church was in the Holy Ghost, nothing could be more practical and nothing more full of intellectual activity. No morbid mysticism had in those days affected it.
The Holy Spirit was in every heart, to make a universal brotherhood, to unite them all in bonds of sympathy and good will...In the beginning, this was the inspiration common to all Christians. It gave to all of them faith, hope, love, courage, patience, submission; it gave them peace in the midst of storms, joy amid trials, life in the hour of death. It was a life within the soul, making all things new."