Some Christians know they are going to heaven. Other Christians are pretty sure they are going there. Many unbelievers, meanwhile, assume they are going to heaven, or they simply don't believe that heaven is even a real place. Try asking 20 Christians and 20 unbelievers about their hope for the afterlife. You will start to see the science behind the assurance of salvation. In other words, there is knowledge to be gained through the systematic study of their responses.
Religion is man's attempt to get to God and to heaven. Christianity is God coming down to man to save him by giving him a relationship with the Lord and one day bringing him to paradise. There is a consistent reality in those who profess faith in Jesus Christ. The ones who say they know they are going to heaven are the same ones who give Jesus all the credit for their eternal life in heaven.