Yeah it's not that surprising when you think about him being a sheltered ultra religious dude who grew up on a small island his entire life and was basically in the public eye since middle school. Probably had little experience with girls or even generally what a normal relationship was like.
Also that was kinda the beginning of "you really think someone would do that, just go on the internet and tell lies?", catfishing and trolling and scamming was not as mainstream as it is now. Of course he should've still known, but it wasn't as ingrained in everyone's minds back then.
I honestly had never heard the term "catfishing" until the Manti Te'o story, and even the concept of it seemed very odd to me because I would think, "how long could ANYONE fall for it if they never got to meet this person face-to-face?"
Well, apparently Manti did, as apparently many others do. I kind of felt bad for him as I watched the doc - he seemed uniquely vulnerable to such a thing, coming from Hawaii to an utterly different environment and culture, where he was basically all alone. And yeah, coming from a very religious family and background probably didn't put him in much contact with shady and duplicitous people (lol, but religion, you say).
At some point, I thought, if this fool had gone to USC like he was supposed to, none of this shit would have happened. He'd have met some co-ed hottie, Samoan or not, and had a normal USC football player's experience.