Just as there are certain truths to know and agree with to be a doctor, or a police man, or a government official, so there are truths to know and agree with to be a Christian. You cannot claim to be a Christian and deny these truths. Jesus gives us several of these crucial truths in our passage.
There are certain essential doctrines that one must believe to be a true Christian. In the conversation with the rich young ruler, Jesus covers several of these essential doctrines. This is crucial for us to understand.