How could God tell Abraham he had never sinned?
God could tell Abraham he had never sinned because Abraham believed God, and that believing was counted unto him for righteousness. God imputed His own righteousness to Abraham, so that in God's sight Abraham was justified. Abraham had done nothing to earn it; he simply believed what God said.
Les explains that when Scripture asks what Abraham believed, the word "it" modifies his believing—his faith itself was counted unto him for righteousness. It was not works or human merit, but simple belief in what God said. On that basis God could look at Abraham and say, "Abram, so far as I'm concerned, you've never sinned. You're justified." Abraham had done nothing; he believed. This illustrates personal salvation: Christ did not just put the human race under an umbrella, but died for every individual, and each person must believe the gospel.