His early education was directed by an uncle, and in his studies, for the ministry, he was assisted by John Abernethy, the leader of the nonsubscribing section of the presbyterians of Ulster.
Duchal began (anonymously) a controversy with Holmes, and the pamphlets which ensued formed the closing passage in a discussion which had agitated Ulster presbyterianism from 1720.
His delicate health and shy disposition kept him out of society; he approves the maxim that 'a man, if possible, should have no enemies, and very few friends' (Sermons, 1762, i.
Late in life, he returned to the study of Hebrew, to test the positions of the Hutchinsonian system, in which he found nothing congenial to his ideas.
He composed aloud, while taking his daily walks, and committed the finished discourse to paper at great speed, in fine crowquill penmanship.
Clarke Irwin states that, in 'Presumptive Arguments for the ... Christian Religion,' he "strongly asserts the Deity of the Lord Jesus Christ".
Andrew Kippis observed that William Leechman plagiarised (1768) the substance and treatment of three sermons by Duchal on the spirit of Christianity (1762).
The original draft of seven sketches, without names, was printed (Christian Moderator, April 1827, p. 431) from a copy by Thomas Drennan; the first three are Michael Bruce (1686–1735), Samuel Haliday, and Abernethy.
Six small volumes, containing 47 autograph sermons by Duchal, 1721–40, which on 18 November 1783 were in possession of William Crawford, D.D., were presented by James Gibson, Q.C., to the library of Magee College, Derry.