The good and wise pastor, Mr. Fuller, is one of the heroes of Guild Court, A London Story. The following passage, late in the novel, strikes me as the answer that MacDonald might have given had someone sincerely asked him, as St. Paul was once asked, "What must I do to be saved?" These are words of advice, not literally from a sermon, and may be found on page 299 of the Sunrise edition, page 346 of the Johannesen edition:
"You must give yourself up to the obedience of his Son entirely and utterly, leaving your salvation to him, troubling yourself nothing about that, but ever seeking to see things as he sees them, and to do things as he would have them done. And for this purpose you must study your New Testament in particular, that you may see the glory of God in the face of Jesus Christ; that receiving him as your master, your teacher, your Savior, you may open your heart to the entrance of his Spirit, the mind that was in him, that so he may save you. Every word of his, if you will but try to obey it, you will find precious beyond what words can say. And he has promised without reserve the Holy Spirit of God to them that ask it. The only salvation is in being filled with the Spirit of God, the mind of Christ."