Why can a person know they're going to heaven when they die?
A person can know they are going to heaven because Christ did everything on their behalf, not because they are deserving. None are deserving of heaven, but believers have God's promises, and Scripture says whosoever shall call upon the name of the Lord shall be saved. This assurance rests on God's promise, not human merit.
Les explains that confession with the mouth unto salvation does not mean a person must tell someone before being saved. Instead, salvation is something to speak about readily, like bragging rights, because of what God has done. He says he knows he is going to heaven, not as braggadocio or because he deserves it, but because Christ did everything on his behalf and he did nothing. None are deserving of God's heaven, yet believers know they are going there because of God's promises. Les also points to the culmination of becoming a believer: recognizing Christ died for our sin, being hopeless in ourselves, and calling upon Him. Verse 13 promises, "whosoever shall call upon the name of the LORD... shall be saved." He illustrates with a lifeguard hearing a drowning child amid noise, suggesting God is the same way.