Lewis points to the modern tendency to believe that the early church swiftly departed from the actual teachings of Jesus. This is convenient for modern man, because then he can suppose whatever he likes to be the actual teaching of Jesus, and discount the bits of what the church teaches that cause him trouble.
But going another direction, we see what Jesus actually did. He gave us a church, against which the gates of hell would not prevail. He gave us His words, both recorded in the Bible and passed down from the disciples, which will not pass away...even though both heaven and earth do. He did, in fact, give us a foundation for our lives. One so secure and unshakable that no matter how scary our lives might get, they will not be destroyed. They're protected by the very God of the universe, who promises to be with us and never forsake us, no matter what.
On that you can rely.
Thank You, God, for being our bedrock.