How can a believer know for sure they will be in heaven?

A believer can know for sure because assurance rests not on personal good works but on the finished work of Christ. He is the propitiation and sacrifice, and His blood in heaven is the eternal testimony of why believers will be in glory. Scripture says you are to know.

Les explains that many people are troubled when a pastor speaks at a believer's funeral as if that person is in heaven, saying nobody can know. But the book says believers are to know. That assurance is not based on what we do in good works or anything else; it is based entirely on the finished work of Christ. He is our propitiation, the sacrifice, our brazen altar, our cleansing, our candlestick, the mercy seat, and the blood of the mercy seat. Because His blood was presented in heaven, it stands as an eternal testimony of why we will be in glory.

On what basis can a believer know they are going to heaven?

Assurance of heaven rests not on good works or anything else a person does, but entirely on the finished work of Christ. He is the propitiation, the sacrifice, and His blood in heaven is the eternal testimony of why believers will be in glory.