Of course, if the Lord sent Moses on an errand, He would not let him go alone. The tremendous risk which it would involve and the great power it would require would render it ridiculous for God to send a poor lone Hebrew to confront the mightiest king in all the world and then leave him to himself. It could not be imagined that a wise God would match poor Moses with Pharaoh and the enormous forces of Egypt. Hence He says, “Certainly I will be with thee,” as if it were out of the question that He would send him alone.
In my case, also, the same rule will hold good. If I go upon the Lord’s errand with a simple reliance upon His power and a single eye to His glory, it is certain that He will be with me. His sending me binds Him to back me up. Is not this enough? What more can I want? If all the angels and arch-angels were with me, I might fail; but if He is with me, I must succeed. Only let me take care that I act worthily toward this promise. Let me not go timidly, halfheartedly, carelessly, presumptuously. What manner of person ought he to be who has God with him! In such company it behoveth me to play the man and, like Moses, go in unto Pharaoh without fear.
[from Faith’s Checkbook by Charles H. Spurgeon]