That would be somewhat difficult, since Arthur was almost certainly born hundreds of years after Jesus, assuming both men were real at all.
The first documented reference to Arthur anywhere in existing knowledge wasn't until 900 AD or so. And nearly every scholar who assumes he was real at all tend to place him somewhere around 400-450 AD in Roman England. Where he would literally have been raised as a Christian himself.
The alternative view would be that he actually descends from unrelated Welsh mythology, but that would likely place him even later in the timeline.
There's pretty much no sane or rational scholar who would ever make the claim that any story specifically about Arthur predates 1 AD. At best, you could argue that Arthur represents specific folkloric archetypes that existed before then, but that's not really the same thing.
Not to mention that Arthurian legends have rather explicitly referenced Christianity at times, which would be a hard thing to do if it preceded it. ---
(\/)(\/)|-| There are precious few at ease / With moral ambiguities / So we act as though they don't exist.