Indeed any lie or error is bad but the degree to which they are bad is not the same. I am not sure what you refer to here about the rapture and will have to go back and read maybe I missed something I have not read the whole topic. To use an example though to make the point about penicion. Say we have two heresy revolving around the topic of the rapture. One is a heresy about the rapture that states that like the rapture will happen tomorrow if you give pastor Bob 20 bucks. It's still bad and is a heresy though because obviously it's going to lead to you joining the cult of Bob and getting defrauded and plodding down a pathway that will accumulates more sins and heresies. Now say for example a heresy about the rapture that is so loopy it goes to the degree that it attacks, undermines, or outright denies the second coming of Jesus, a key point in the Gospel, and instead states that it's Pastor Bob that's going to come again and rapture everyone and that where the Bible says Jesus is coming back they made an error in millennia of tradition, the writers actually meant Bob (and if this sounds absurdist to you, actually many cults have basically run this exact script). For falling for the Bob's rapture scam heresy it's bad still, but it's you're getting scammed however long you want to get run through the ringer is up to your obstinacy at that point. The second coming of Bob rapture heresy is more pernicious and would be judged as certainly damnable for adhering to it by reason it denies a fundamental truth about God himself and a key point of the Gospel of Jesus to the point that it's almost just a wholly separate pagan religion, and basically the Second Coming of Bob adherent is basically be turning their back on God in favor of Bob and in truth Bob ain't gonna save em, only Jesus can do that.
Again, the damnable heresies all attack God basically, anything contrary to the trinity doctrine as best outlined in the Athanasian Creed or anything that attacks the key points of the Gospel of Jesus (virgin conception, birth, life and ministry, crucifixion, resurrection, ascencion and session, and promise to come again) are the damnable heresies. Holding to a heresy is bad regardless of the severity and will lead to the path of errors, but holding to a damnable heresy is a guarantee to the hellfire by reason that it's a direct attack or rejection of the only true God.