For starters, Jong's character isn't a regular Christian.  He's a guy (an immortal) who actually MET Jesus (who was also an immortal), and the "type" of Christianity Jong was trying to convert Marcus into wasn't one of the 600 or so different kinds of 2nd-century christianity but essentially Jong's "Religion of One".  When the dialogue scenes were played out between him and Marcus (who, I will note, is a child prodigy; those of you who know Roman history know just WHO Marcus turns out to be), Jong wasn't even able to clearly express what the central points of his character's ideology were to Marcus, ie. precisely what the fuck he wanted Marcus to believe in?! He didn't tell Marcus that he (or Jesus) were immortals, he didn't make it clear about the resurrection, or any particular points of dogma other than this kind of general mysticism and "christians aren't bad people" thing.

The result was that the now-adult Marcus is remarkably open toward christian beliefs, rejects the persecution of the Christians, and, in combination with the training he received from the finest living philosophers of the empire (thanks to the Emperor Hadrian's infatuation with him), Marcus has a deeply investigative and tolerant open mind.  He has been shown in the campaign acting on his own initiative to read other "forbidden" christian texts (beside the one Jong wrote), and has expressed a general sympathy for christianity without being willing to reject altogether the existence of the Roman gods.

So the question here really is: What the fuck did Jong want to happen? If his goal was to change history, he did. Marcus (the heir to the imperial throne) is now sympathetic toward Christianity, in a way he never clearly was historically.  Was Marcus going to convert whole-hog to Christianity? Not the way Jong handled things he wasn't.  Jong didn't make him a part of a church, didn't surround him with Christian teachers, and didn't block Marcus from outside influences (namely the Stoic teachers that Hadrian provided for him).  The problem is, really, that Jong was never very clear about what he wanted or just how to get it. And he seemed to have two very conflicting desires for young Marcus; on the one hand for Marcus to become a model Roman citizen, and on the other to become a believing and good Christian.  The biggest problem with this is that, at this point in the campaign, those two things are diametric opposites, and Jong is the ONLY PERSON ON EARTH who thinks they don't have to be.  Every other Christian around who has the courage of their convictions is willing to die rather than to worship the Roman State Gods, or if they don't they know they're doing "wrong" by failing to martyr themselves.  Jong wants a Christianity that somehow retains its monotheism but is acceptable to Roman civilization, and that just can't happen yet.

So in essence, he raised Marcus to have a choice between being the dutiful Roman patrician and a follower of the religion of his father (and I mean literally the religion of his father, like I said before, Jong doesn't belong to any organized church and no one else believes in the christianity he believes in).  He sees Jong's character say he's a Christian, but Jong's christianity doesn't seem to mean anything other than to say "yeah, Jesus rocked".  He's certainly not willing to lose all his wealth and privilege for being Christian, or stop being a Patrician, or stop killing people, or stop worshipping at the Pantheon, or stop anything else that makes him a model Roman.  So in a very real sense, Marcus is following in his adopted father's footsteps, with the one little difference being that he's not quite willing to even call himself a Christian, in part because of his studies of the rest of Christianity, and in part because of his Stoic training, and in part because of his legendary honesty.

Its basically a case of Jong telling Marcus "Do as I say, not as I do" or something like that, and poor Marcus not really knowing what the flying fuck Jong actually wants from him.  A declaration? A statement? I mean, let's hypothetically say that Marcus said to Jong "ok yes, I'm a Christian".  I ask you Jong: What the hell would happen AFTER that? In what way would you want Marcus to act differently than he does now?

Because the thing is, Jong, Marcus thinks admires you (as in Quintus, your character) greatly for many things, but he also thinks that Quintus is something of a hypocrite.  And Marcus "Verissimus" Verus Quintus Antoninus can NOT be a hypocrite, its not in him.  For him to declare himself a Christian, he'd have to figure out what exactly it'd mean to be a Christian (which you never really managed to explain to him very well), and he'd have to be upfront and honest about BEING a Christian. So if you taught him that there's only one God, that honesty would mean no longer worshipping at the temple of Jove or the Divine Trajan or anyone else, and it would mean dying in the Lion Pen at the Games. Marcus isn't sure he is a Christian or not, but he knows that if he was one, it would be more than just saying that in secret to you, and then doing nothing different about it and going on acting like a model Roman-pagan.

"The character/player couldn't explain it" is a pretty weak excuse--not everyone has a Degree in Religious Studies, you know? Some of us have to wing it with what we remember from game sessions that happened a year ago, and by spending a feat for Knowledge (Religion). This isn't Amber Diceless, no matter how much it looks like it at times.