A version was published by Yale University Press in 2009 as Kinship by Covenant: A Canonical Approach to the Fulfillment of God's Saving Promises.

[3] Harvard professor Jon D. Levenson described it as "a learned and well-written volume interprets covenant as the red thread running through both testaments of the Christian Bible.

He was bothered that the Catholic Church was the only tradition that upheld the ancient teaching of prohibiting contraception that Protestants abandoned in the early 20th century, such as at the 1930 Lambeth Conference.

Hahn continued to study various issues relating to salvation, faith, and good works, as well as the Protestant doctrine of sola scriptura.

This new family, according to Hahn, is headed by Christ and the Pope is his "prime minister" to whom he has given the keys of the kingdom, a process that he believes is also present in the Old Testament.

[8] Hahn's wife, Kimberly, had a similar conversion at a slightly later date, entering the Catholic Church at Easter 1990 in Joliet, Illinois.

A popular speaker,[11][12] Hahn has given over 800 talks in the US and other countries on theological and biblical topics related to the Catholic faith and appears regularly on the Eternal Word Television Network (EWTN).

[13] Since 1990, Hahn has taught at the Franciscan University of Steubenville, where he is the Father Michael Scanlan, TOR, Chair of Biblical Theology and the New Evangelization.

[15] In 2014, Francis Cardinal George appointed Hahn to the newly established McEssy Distinguished Visiting Professorship in Biblical Theology at Mundelein Seminary in Chicago.