When you assert that God exists and Jesus is his earthly representative and that only through belief in Jesus can you reach heaven and enjoy eternal life, you have to jump the following cognitive hurdles:
- you have to believe in the supernatural
- you have to believe that there are supernatural entities that persist that can interact with our reality in a meaningful way
- you have to believe that one supernatural entity exists that is responsible for the creation of our natural world, is capable of and in fact does intervene with us in response to our prayers or in positive or negative response to our belief in it and our adherence to its written or unwritten rules
- you have to believe that the Old Testament describes this being - which we'll call "God" - and that this conception and the words that describe its rules and expectations supercede all other written and unwritten descriptions and conceptions of an ultimate supernatural being throughout the existence of the universe.
- you have to believe that an addendum to the Old Testament - the "New Testament" - describes an earthly representative of God called Jesus, whose words, transmitted to us through intermediaries whom we know little or nothing about, modify and supercede the basic conceptions laid down in the Old Testament, and that these words and rules must be followed in order to achieve life after death and eternal existence in a place called Heaven while the vast preponderance of humans who ever lived and/or who do not believe this particular set of words and guidelines will suffer eternal torment
- you have to believe that God will eternally maintain both Heaven and Hell for all eternity to reward and/or punish the aforesaid for a few decades of existence in the real world and the adherence to and belief in the written words and concepts
In polite conversation, the response to this is usually "Are you insane"? A less polite response - which I will convey to you in the interest of completeness - is "ARE YOU F**KING INSANE???"
When you try to convince another person that they should jump the cognitive hurdles that we outlined above, and that they should not inquire of your sanity but should instead accept your assertion at face value - you'd be well served to bring evidence that each hurdle that we described above is in fact an accurate representation of the world that can be affirmed through repeated, systematic observation, and not just some insane f**king sh*t someone made up.
That's putting it politely.