He was thought to show a leaning towards Socinianism and Arminianism.
If Socinianism had challenged natural theology - Christ, according to it, was the prophet who first revealed the way to eternal life - it had glorified the natural powers of man; and the learning of the Arminian divines (friends of Grotius and Locke) had helped to modernize Christian apologetics upon rational lines.
One of the first acts of the new church system was to excommunicate Erastus on a charge of Socinianism, founded on his correspondence with Transylvania.
In 1767 he was appointed to the charge of Mill Hill Chapel at Leeds, where he again changed his religious opinions from a loose Arianism to definite Socinianism and wrote many political tracts hostile to the attitude of the government towards the American colonies.
It may be added that after the Reformation Arianism was revived in Socinianism, and Pelagianism in Arminianism; but the conception of heresy in Protestantism demands subsequent notice.
Before taking orders in 1658 he was in the habit of preaching as the champion of Calvinism against Socinianism and Arminianism.
The charge of Socinianism was frequently brought against him, but, as Tillotson thought, "for no other cause but his worthy and successful attempts to make the Christian religion reasonable."
In Socinianism (see below) we have perhaps the only instance of humanistic antecedents leading to the formation of a religious sect.
From the standpoint of the history of enlightenment, as Harnack has observed, " Socinianism with its systematic criticism (tentative and imperfect as it may now seem) and its rejection of all the assumptions based upon mere ecclesiastical tradition, can scarcely be rated too highly.
Ere long Arianism and Socinianism were general among English Presbyterians (see Unitarianism).
His suspected Socinianism was the cause, it is said, of his exclusion from the chair of dogmatic theology.
Socinianism taught a new spiritual body, an intermediate state in which the soul is near non-existence, an annihilation of the wicked, as immortality is the gift of God.
The doctrine of conditional immortality taught by Socinianism was accepted by Archbishop Whately, and has been most persistently advocated by Edward White, who "maintains that immortality is a truth, not of reason, but of revelation, a gift of God" bestowed only on believers in Christ; but he admits a continued probation after death for such as have not hardened their hearts by a rejection of Christ.
A Glasgow professor, the Rev. Mr Simson, was attacked for Arminianism and Socinianism as early as 1717; and the battle raged between the more severe Presbyterians - who still hankered after the Covenant, approved of an old work The Marrow of Modern Divinity (1646), and were especially convinced that preachers must be elected by the people - and the Moderates, who saw that the Covenant was an anachronism, thought conduct more important than Calvinistic convictions, and supported in the General Assembly the candidates selected by patrons, as against those chosen by the popular voice.
The surviving church became involved in Socinianism and Universalism, but maintained a somewhat vigorous life and, through Wickenden and others, exerted considerable influence at Newport, in Connecticut, New York and elsewhere.
The withdrawal of members to form other churches in the neighbourhood and the intrusion of Socinianism almost extinguished the Charleston church about 1746.
But after his stay at Malta, Coleridge announced to his friends that he had given up his "Socinianism" (of which ever afterwards he spoke with asperity), professing a return to Christian faith, though still putting on it a mystical construction, as when he told Crabb Robinson that "Jesus Christ was a Platonic philosopher."