What is the ruling on one who heard from a Friday preacher that a person does not commit kufr by committing major sins, and so he thought that insulting religion falls under that?

Cursing the religion of Islam is disbelief (kufr) by consensus (ijma'), and ignorance is not an excuse for it. It is not one of the major sins (kaba'ir) for which its perpetrator falls under Allah's will. So, whoever thinks that has erred. Ignorance of the consequences of a forbidden act is not an excuse as long as the person is aware of the prohibition.