Is it wrong for a pastor to preach at a believer's funeral as though he knows the person is in heaven?
No, it is not wrong for a pastor to preach at a believer's funeral with assurance of heaven. Scripture teaches believers are to know their eternal destiny, not based on works but on Christ's finished work as propitiation and sacrifice. The blood of Christ is an eternal testimony of why believers will be in glory.
Les explains that some people attend a believer's funeral and come away disgusted because the pastor preached as if he knew the deceased is in heaven, objecting that nobody knows. But Les says that is not what the book says; the book says you are to know. This assurance is not based on what we do in good works but entirely on Christ's finished work. Christ is our propitiation, the sacrifice, our brazen altar, our cleansing, our candlestick, the mercy seat, and the blood of the mercy seat. The blood He presented in heaven is an eternal testimony of why believers will be in glory, so a pastor can preach with that confidence.