"'Forgive us our sins as we forgive those that sin against us.'"

Lewis is quoting the Lord's Prayer, which is found in Matthew 6 and Luke 11 ("forgive us our sins, for we ourselves forgive everyone who is indebted to us").

After completing the prayer in Matthew 6, Christ explains, "For if you forgive others their trespasses, your heavenly Father will also forgive you,but if you do not forgive others their trespasses, neither will your Father forgive your trespasses" (verses 14-15).