People are not well versed in theology these days so his protestant church just loved this teaching. It was quite a while before anyone pointed out that this is precisely the opposite of what the reformers said about justification. So he started to read. He read Alistair McGrath's book on the reformation where he said this idea, which he called forensic justification, was completely new around the year 1500. Maybe you can find it in Jan Hus but not earlier. He started reading major Christian thinkers before that and saw that this was true.
The interesting thing is he did all this without any influence from Catholics and any notion of becoming Catholic. He did eventually convert but it took him a long time. He didn't even want to refer to the pre-reformation Christians as Catholics. He called them historical Christians or orthodox Christians or some such phrase.
While Exodus is an interesting angle from which to approach the justification from it is not as strange as I first thought. Paul and the other apostles were Jews. They knew the Old Testament first. So they would approach everything starting there. Paul explicitly draws the same parallels between our Christian journey and the journey in Exodus.
The other point he brings up is how the implausibility of forensic justification has led some to reject Christianity altogether. He spent a lot of time arguing with atheists and heard this often. Why should a good man go to hell because he believed the wrong thing and a bad man go to heaven because he had said the sinners prayer at some revival once? Is that really fair? I know grace is inherently not fair but any God that declares someone righteous when they are not seems quite strange.
For me, it teaches me the value of using ordinary layman language. When we are talking about heaven and hell there can be no more important topic to anyone. If they are real and our lives determine which one we go to then we need to be very concerned. Yet finding the real information in the midst of all the falsehoods is quite a challenge. Really impossible without God's help. I can see people saying God would not leave us like that so He must not exist. I can see people saying God would not leave us like that so Catholicism must be true. To say God did leave us like that seems like the only choice a Protestant has.