Certainty is the mark of the common-sense life: gracious uncertainty is the mark of the spiritual life. To be certain of God means that we are uncertain in all our ways, we do not know what a day may bring forth. This is generally said with a sigh of sadness, it should be rather an expression of breathless expectation. We are uncertain of the next step, but we are certain of God. Immediately we abandon to God, and do the duty that lies nearest, He packs our life with surprises all the time.

My Utmost for His Highest (1956). Section "April 30".