Jesus Christ could come down off the cross, tell them Biden legitimately won, and they would not believe Him.
Two Trump voters, Joe and Bob, were out fishing when their boat overturned and they unfortunately drowned. They were devout Christians and went to Heaven. After being admitted by St. Peter they were greeted personally by God, who asked if there was anything that they wanted to know about their time on Earth.
Joe spoke up and asked, "Yes, Heavenly Father. We know in our heart of hearts that President Trump was actually reelected, and that Joe Biden only won because of voter fraud and cheating by the Democrats. Can you tell us how much President Trump actually won by?"
God replied, "My son, this may disappoint you, but I must tell you that there was no significant voter fraud or other cheating. Joe Biden won a free and fair election."
Joe turned to Bob and said, "Can you believe this? The conspiracy went higher than we ever imagined!"