What are the serious sins that can cause Gods chastisement. I believe the two most serious sins are nothing to do with the flesh, sinning, living in sin, things like that. The serious sins are spiritual in nature.
But fornication has its special place with that it can damage the body. Still the worst sins a christian can commit seems to be spiritual in nature rather than carnal.
Changing the word of god
It seems to be a serious sin to mess with the scripture, the word of god, which is the king james bible also called AV1611.
Things like adding to it or taking away from it or perverting it or trying to make a unity with the modern versions. Trying to say that the king james bible and the modern versions and the paraphrases are really all saying the same thing.
Hating the jews
Here i would say that replacement theology is the beginning of it. But I think when people start hating the jews or hating the nation of israel or claiming that the ethnic jews are not descendants of Jacob but rather are the khazars. People who are antisemitic, that is work actively to promote hate against the ethnic jews. These kind of things. It is a spiritual sin.