Let me begin with the most-deceptive elements. Scott Hahn begins by describing himself as a hardcore Calvinist in seminary. What he doesn't explain is why he chose to go to Gordon-Conwell Seminary, which is evangelical, but not Reformed. However, the answer becomes evident, even if unspoken. On page 31, Hahn informs us that he had come to "discover" that Saint Paul did not teach the doctrine of sola fide (justification by faith alone). Any Protestant reading that should be struck dumb, because we immediately think of such Pauline references as Romans 4:5, "To the one who does not work, but believes in Him who justifies the ungodly, his faith is counted as righteousness." Then Hahn adds his assertion that sola fide isn't taught anywhere in Scripture, claiming James 2:24 as his support. The reason I make such a point of this is to demonstrate that Hahn had abandoned Protestantism long before his official conversion.
We see the reason for this when Hahn mentions that he had become involved with Norman Shepherd, who was forced to leave Westminster Seminary for undermining the biblical view of justification. Hahn even mentions Shepherd's heresy trial, demonstrating that he was well-aware that those views were outside the pale of Reformed doctrine. Shepherd was the father of the Federal Vision movement, which has disrupted Reformed churches with an attempt to bring back this exact Catholic doctrine of justification. Hahn went to his first pulpit with these views, at a Federal Vision church, but does not say that he informed his presbytery of his views, a deceptive act. While there, he also began to teach at a tiny seminary, i.e., in spite of the "major seminary" claim of one article promoting him. Again, we see that this "Presbyterian" minister was never really Presbyterian.
In addition to his justification heresy, Hahn began the ministry with erroneous views on the sacraments. He complains, p. 49, about the Protestant doctrine of the Lord's Supper - or Eucharist, as he prefers to say - that it is a mere symbol. This is repeated on page 56. Unfortunately for him, that is not the Protestant view, at least among Lutherans, who hold to consubstantiation, or the Reformed, who hold to a spiritual, noncorporeal Real Presence. As the Westminster Confession of Faith XXIX:7 says, "Worthy receivers outwardly partaking of the visible elements in this sacrament, do then also, inwardly by faith, really and indeed, yet not carnally and corporally, but spiritually, receive and feed upon Christ crucified, and all benefits of His death; the body and blood of Christ being then, not corporally or carnally, in, with, or under the bread and wine; yet, as really, but spiritually, present to the faith of believers in that ordinance, as the elements themselves are to their outward senses." Which solution would be worse: that Hahn was ignorant of this doctrine? or that he was knowingly railing against a doctrinal strawman?
One of the endorsements on the back cover of the book says, "This book is inspiring and exciting for any reader desirous of rediscovering, from a scripture base, the reasons why the Catholic Church believes, teaches, and practices its doctrines." But this is exactly what the book doesn't do.We read statements such as, "I had fallen head over heels in love with the Lord in the Eucharist" (p. 88), and "I felt the Lord unleash his power through his mother" (p. 89). This is sentiment, not biblical exegesis. He also mentions direct revelations "from the Lord" on pages 114 and 115. Voices in the head are, again, not biblical exegesis.
This book, as well as other publications from Hahn, are promoted by (some, not all) the Catholic Church in an effort to convert Protestants. You can see them at his website, linked at the top, as well as here. This is what plays into the Acts 20:30 reference above; Hahn hasn't just committed apostasy himself, with his family, he wants to convince the rest of us evangelicals to follow him! God's warning to us is found in Deuteronomy 7:4, "[He] would turn away your sons from following Me, to serve other gods. Then the anger of the Lord would be kindled against you, and He would destroy you quickly." And God's warning to Hahn is found in Deuteronomy 13:5, "That prophet shall be put to death, because he has taught rebellion against the Lord your God."
A video by two ex-Catholic priests with a viewpoint opposed to Hahn's can be seen here.