I’m not exactly sure where to put the line on how much you have to understand about Christianity before rejecting what you think it is is actually rejecting it itself. But I think we would both agree that there is one - if someone in the middle of a jungle somewhere hears that some people think that because someone was executed a long time ago, good stuff can happen (and nothing else), that amount of knowledge about what the word Christianity means is not enough to damn someone if they don’t accept it as true.
Likewise, if someone has been utterly convinced (through no fault of his own) that the word Christianity refers and only refers to, say, beliefs of the Westboro Baptist variety, and knows no other alternatives, I think we would probably agree that for that person to reject what he thinks the word means would not only not be damning, but would be good.
But aside from extremes, I don’t know how much ignorance or false ideas would reduce culpability for rejecting the faith by how much.
It is dogma that that Mary was born without original sin. I can be misinformed about this dogma, without being so because I reject Christ. I can simply not understand why it is true, and not understand that I need to accept it despite this (well, I can’t, as I am now, but someone could) and still be trying to follow Christ while in my head thinking that the dogma of the Immaculate Conception is false.
I would be wrong. Gravely so. But it might not be something I was culpable for.
I can not know who Jesus is, and reject all false notions of Him that I have been presented. In this case I am not rejecting Christ, but rejecting falsehoods, true. But since I don’t know who Jesus is, I am not explicitly accepting Him either. And so, because I do not even know who He is, I would verbally reject the dogmatic statement that God became Man, because I don’t know what the word God refers to and do not explicitly realize even that there is a reference for the word “God” - even while at the same time doing my best to seek goodness and truth.
That is, I can explicitly say that I reject any sentence involving the word "God: (God became Man, God is Triune) because I don’t know who God is but without actually rejecting God (Truth, Goodness) Himself. And again, not being culpable for this requires confusion on my part.
Of course you might say that rejecting false conceptions of dogma doesn’t count as rejecting dogma (so isn’t mortal sin), and that if we do not have any idea what the true dogma actually is (through no fault of our own), then by failing to accept it (through no fault of our own) we are not necessarily committing mortal sin. This would, I think, be an equivalent way of phrasing my point.
Nothing more needs to be said!