Do not be Deceived, Wormwood...

C S Lewis said of MacDonald's Unspoken Sermons, "My own debt to this book is almost as great as one man can owe to another," and what seems a clear example of this indebtedness can be found in The Screwtape Letters. Screwtape's warning about the kind of faith which relies upon will rather than feelings bears a strong resemblance to another faith-passage from The Child in the Midst. This latter would also appear as the very first entry in Lewis's George MacDonald: An Anthology.

Do not be deceived, Wormwood. Our cause is never more in danger than when a human, no longer desiring, but still intending, to do our Enemy’s will, looks round upon a universe from which every trace of Him seems to have vanished, and asks why he has been forsaken, and still obeys.
— The Screwtape Letters, Letter 8
That man is perfect in faith who can come to God in the utter dearth of his feelings and desires, without a glow or an aspiration, with the weight of low thoughts, failures, neglects, and wandering forgetfulness, and say to Him, ‘Thou art my refuge.’
— Unspoken Sermons, First Series, The Child in the Midst