For them to become afraid now, meant that they did not believe they were going to make it. They did not have faith in the words of the very God whom they were following. Now Jesus understood their lack of faith, but He did not say it was alright. He gently, yet firmly admonished them. This is not just a New Testament concept.
Faith is important.
In other words, no maturity without perseverance. We must have faith in the whole Word of God in order to become mature Christians. Otherwise we will never attain to the whole measure of the fullness of Christ. There are no neutrals. It is lack of knowledge of the Word which keeps us weak.