Why would adding works to salvation make God a liar?
Adding works denies that God has already done everything required for salvation. If God declares salvation complete through Jesus and a person insists their own deeds must supplement it, that contradicts God's own Word and makes His statement untrue. Scripture says justification comes to the one who believes in Jesus, not to those adding works.
Les explains that God will not allow any human being to add to what He has done. It is against His nature and would make Him a liar. If God says He has done everything needed for salvation, and a man comes along saying he must still do this or that, he is effectively treating God's provision as insufficient. Les points out that justification comes to the one who believes in Jesus, which Paul connects with His death, burial, and resurrection. This is tied to reconciliation: God does whatever the sinner needs done on his behalf to bring him back to Himself. Les warns that adding works puts a person on thin ice because it is just not in Scripture. The reconciled believer then has a ministry of reconciliation.