From this link:
The Arian controversy of his time left its mark on Watts. His hymns contain an entire book of doxologies modelled on the Gloria Patri. But at the conference about the ministers at Exeter held at Salters' Hall (1719) he voted with the minority, who refused to impose acceptance of the doctrine of the Trinity on the independent ministers. He did not believe it necessary to salvation; the creed of Constantinople had become to him only a human explication of the mystery of the divine Godhead; and he had himself adopted another explication, which he hoped might heal the breach between Arianism and the faith of the church. He broached this theory in 'The Christian Doctrine of the Trinity' (1722), and supported it in 'Dissertations relating to the Christian Doctrine of the Trinity' (1724-5). He returned to the subject in 'The Glory of Christ as God-Man Unveiled' (1746), and 'Useful and Important Questions concerning Jesus, the Son of God' (1746). His theory, held also by Henry More, Robert Fleming, and Burnet (DORNER,The Person of Christ, div. ii. ii. 329, transl. Clark), was that the human soul of Christ had been created anterior to the creation of the world, and united to the divine principle in the Godhead known as the Sophia or Logos (only a short step from Arianism, and with some affinity to Sabellianism); and that the personality of the Holy Ghost was figurative rather than proper or literal. None of the extant writings of Watts advances further than this; but a very pathetic piece, entitled 'A Solemn Address to the Great and Ever Blessed God' (published in a pamphlet called 'A Faithful Inquiry after the Ancient and Original Doctrine of the Trinity' in 1745, but suppressed by Watts at that time, and published in 1802), shows how deeply his mind was perplexed and troubled. He lays out all the perplexity before God, stating his belief in too very words of Scripture generally, with the plea 'Forbid it, oh! My God, that I should ever be so unhappy as to unglorify my Father, my Saviour, or my Sanctifier. . . . Help me . . . for I am quite tired and weary of these human explainings, so various and uncertain.' Lardner affirmed that in his last years (not more than two years at most, in failing health) Watts passed to the unitarian position, and wrote in defence of it; the papers were, as Lardner owned, unfit for publication, and as such were destroyed by Doddridge and Jennings, the literary trustees. Lardner declared also that the last belief of Watts was 'completely unitarian' (BELSHAM, Memoirs of Theophilus Lindsey, pp. 161-4). The testimony, however, of those who were most intimate with Watts to his last hours is entirely silent as to any such change; and his dependence at death on the atonement (which is incompatible with 'complete unitarianism') is emphatically attested (MILNER, Life, p. 315).