William Collins received a thorough education, graduating B.D. and touring Europe prior to his call to serve at Petty France. In a funeral sermon preached by John Piggott, a fortnight after Collins' death on 30 October, 1702, mention is made of the encouraging "Offers he had to join the National Church, which he judiciously refus'd; for 'twas Conscience, not Humour, that made him a Dissenter". The esteem in which he was held by his brethren may be noted in the fact that he was requested by the General Assembly to draw up a Catechism, and on the strength of this Joseph Ivimey asserts "it is probable that the Baptist Catechism was complied by Mr. Collins, though it has by some means of other been called Keach's Catechism". Later in his work, Ivimey transcribes a letter from Collins to Andrew Gifford, pastor of the Pithay Church in Bristol, and arguably the most important Particular Baptist outside of London. In the letter, Collins refers to the latest impression of the Catechism, and states that there are "some thousands left".

Collins, according to Piggott, "was a studious elder and a good pastor, noted for his peacable spirit. The Subjects he ordinarily insisted on in the Course of his Ministry, were the great and important Truths of the Gospel, which he handled with great Judgment and Clearness. How would he open the Miseries of the Fall! And in how moving a manner would he discourse of the Excellency of Christ, and the Virtues of his Blood, and his willingness to save poor awaken'd burdned [sic.] Sinners!... His sermons were useful under the Influence of Divine Grace, to convert and edify, to enlighten and establish, being drawn from the Fountain of Truth, the Sacred Scriptures, with which he constantly convers'd in their Original Languages, having read the best Criticks, Antient and Modern; so that Men of the greatest Penetration might learn from his Pulpit-Discourses, as well as those of the meanest Capacity".

Such a testimony of his character and abilities well suits one thought to be co-editor (along with Nehemiah Coxe) of the Confession of Faith (see Documentary Sources and Origins of the Confession) and one to represent his church and subscribe it.