There, he met other radicals, including Hubert Stogdon, and achieved a reputation for rejecting human authority in matters of religious controversy, belief, and practical piety, privileging what he believed to be the indubitable consequences of reason and argument over passive faith and received wisdom.
His Essay on Fundamentals showed the influence of Samuel Clarke's Scripture Doctrine, the work which had triggered the Exeter Controversy, and which Hallett's students, most notably Stogdon, had encountered and read secretly several years earlier.
Like Clarke, Foster hinted at the non-essentiality of the doctrine of the Trinity; he argued that the fundamentals of the Christian faith should operate in a marriage between reasonable interpretation of natural and revealed religion.
By this point his Socinian leanings were well known; he and Burroughs were the only London ministers to invite the disgraced and formerly imprisoned Arian heretic Thomas Emlyn to preach.
Despite criticism from many orthodox Calvinists, Foster was becoming a celebrated preacher and academic, winning increasing recognition from many moderates within Protestant and Roman Catholic dissent, as well as from within the radical Deist wing of the Church of England.
He was awarded the degree of Doctor of Divinity (DD) from the Marischal College in Aberdeen in December 1748 and was on several occasions offered livings in the conformist Church of Ireland by Bishop Thomas Rundle.
Alexander Pope wrote in one of his Satires: He also attracted freethinkers and London wits to his Old Jewry meetings and was respected - though disliked - by the orthodox Congregationalist Philip Doddridge of Northampton.