Actually, much of the New Testament is a distortion of the Hebrew Bible, and Jewish scholars have been trying in vain to point that out for centuries. Although I have no formal training in Biblical studies, I've been well aware of that distortion for over thirty years. So I can see for myself that Jewish scholars are either more knowledgeable about their Bible than Christian scholars, more honest about their Bible than Christian scholars, or both.
So in general the people who originated a religion, I think it's fair to say, are more knowledgeable about that religion than people from other cultures. For example, Christian scholars probably know more about Christianity than Jewish scholars, Muslim scholars, or scholars who are atheists do. This is a simple fact that should be obvious, but since you wish to challenge it, allow me to point out that if a scholar is a Christian, then she or he may have been raised in the Christian faith and has known it intimately while scholars who are not Christian are less likely to have such first-hand experience with the Christian faith.
Speakers of modern English can scarcely understand Chaucer's late middle English of six centuries ago. Modern Catholics most likely would barely even recognize a medieval mass. That's the most important and central rite of an institution with unbroken continuity over a mere thousand years. Judaism by contrast has for nearly two thousand years existed as scattered communities often disrupted by persecution - it borders on the miraculous that it has survived at all. The religion of modern Jews is not even the same as the religion of Jews from the second or third century... let alone the religion of Jews from a couple of centuries earlier, when the all-important temple served as the focal point and site of sacrifice, forgiveness and faith... and let alone the religion of Isaiah from a further seven centuries before that
, prior to Greek and Persian (particularly Zoroastrian) influence and without even a bible to read!
In cases where the time gap and evolution of language, culture and customs has not been so great, such as medieval Catholicism then, yes, there may occasionally be times when a childhood background in a more modern version of the 'same' religion affords a scrap of knowledge or insight that might, perhaps, be missed by someone with only formal study: A decade of formal study and professional work on Christian history will provide far, far more knowledge than twenty years of casual childhood church attendance, but there is of course the possibility
of the latter catching something that the former misses. But on the flip side of the coin such a childhood background carries with it all sorts of biases, from more overt issues like the temptation to gloss over the negative and highlight the positive aspects of that earlier and less sophisticated version of the Catholic faith, to more subtle tendencies such as viewing the medieval version as trending towards the familiar. The former benefits - those rare glimpses of insight which might be missed by formal education - are increasingly not
likely to be missed by lifelong scholars since (particularly in the field of biblical studies) those insights will almost certainly be published by someone and picked up by others. On the other hand the biases are both much more likely to be an issue for scholars from a background of faith and, being subtler, also have a greater chance of influencing their work.
That's not say that religious scholars can't do excellent work studying the history of their own faith - and undoubtedly have some advantages over fresh-faced students just out of a few years in college - but if anything, compared to longtime scholars from a different culture whose interests are merely academic and objective, religious scholars' work is done in spite of
their background and bias, not because of it.