Though it was historically attributed to Augustine, this letter was actually written by Fulgentius of Ruspe in the early 6th century. It appears in English in volume 95 of the series The Fathers of the Church, page 59. The introduction there notes the historical context and briefly discusses the authorship debate.
In answer to Peter's request [for a basic doctrinal guidebook], Fulgentius discusses the teaching of the Church on the Trinity, the Incarnation, Creation, the Fall, Redemption, the sacraments of Baptism and Penance, and Marriage and Virginity. While most believe that Fulgentius is the author of the entire work, there have been some who have posited a second author for the second half which constitutes a recapitulation of the first half with each section beginning: "Hold most firmly and never doubt...."
The Latin text is in Migne, Patrologia latina, volume 65, 671ff.