He headed the commission that eventually published the 1928 Book of Common Prayer (BCP), which governed worship in Episcopal churches until 1979.

Although he rejected the label of "liberal," his amendments to the Anglican liturgy steered the Episcopal Church away from the doctrines of original sin and total depravity, and (if tentatively) towards gender equality.

He then graduated summa cum laude from Harvard College in 1891, and obtained a bachelor's of divinity from Episcopal Theological School in 1894.

[2][8] An active parish priest, Slattery reportedly wrote over 2,500 personal notes to his parishioners a year.

[11][12] In 1919, Slattery applied for the position of Bishop of New York, but lost the election to Charles Sumner Burch.

[17] In 1922, following the death of Cortlandt Whitehead, Slattery became the chairman of the Episcopal Church commission tasked with revising the 1892 Book of Common Prayer, which had changed "little of substance" from the 1789 edition, and whose liturgy and catechism were considered theologically conservative.

[21] According to Henry Washburn, dean of the Episcopal Theological School, "[t]o Slattery, more than to any one else, the general features of the revision are due.

When the commission was convened, it was instructed to refuse to "consider or report[]" any "proposition involving the Faith and Doctrine of the Church.

"[25] Although Slattery rejected the label of "liberal," he took the modernist side of the Fundamentalist-Modernist Controversy; shortly before publishing the 1928 BCP, he declared that the Episcopal Church has "no fundamentalists in the sense in which that word is used to-day.

"[26] The commission's revisions made "far-reaching, and in some instances radical," changes to both language and theology, decisively moving away from the concept of total depravity.