Will God ever say that trusting Christ wasn't what it seemed to be?
No. God will never disappoint the one who trusts Christ. Christ claimed to have done everything necessary, and He is faithful. The blood of Christ is faithful, so the believer's confidence rests on Christ's faithfulness; God will not disappoint that trust.
Les explains this with the chair illustration: people sat down without inspecting the chair, exercising faith that it would hold them, and for the chair to be trustworthy it had to be faithful. Likewise, Christ claimed to have done everything necessary, and faith is placed in that. Les says that when he gets to the eternal abode, God will not say, "I'm sorry, Les, but it wasn't what it seemed to be." Instead, He says the blood of Christ is faithful; He is faithful, and that is why we can trust Him. This is what Paul is driving at: it is through the faithfulness of the One who is faithful to keep us, because He loved me and gave Himself for me. Les also mentions his nine-year-old grandson asking whether he loved God enough or had enough faith not to be left behind; the point is that it is not how much we love Him, but how much He has loved us.