Moses instructs us regarding our feelings about ourselves. We often claim that we have no talents, just as Moses said, "I am not a man of words." He did not have the gift by nature, and he had not developed it since God began speaking to him. This is the same man about whom Stephen, while defending himself against the Jews, said “was mighty in words and deeds” (Acts 7:22).
There is no contradiction here. Both Moses and Stephen were correct. Moses did not have the gift of speaking. The power in Moses' words was not in himself, but in what God added to his words. God made the impact on the hearer's mind.
It could be speculated that Moses never really overcame this, never becoming eloquent as men would count eloquence. Yet, what he said had awesome power because God was in what he said. Both men were correct. Moses said powerful things because God added to what he said.
This is instructive for us because we are similar. We tend to ask, "Who am I?" or "What can I do?" The answer is that God has called the weak of the world (I Corinthians 1:26-31), and all we have to offer Him is our lives and a willingness to be used. He adds where we lack. He does this so that no man can glory in His presence. God intends this to humble us. We have to recognize that God adds the increase and makes effective what we say and do.
Moses undoubtedly had learning from his upbringing in Egypt that was as good as a person could receive at that time. He had ideas about what a leader should be like, that a hero needed to be a blazing personality who commanded peoples' attention, who was good-looking and had everything going for him.
God does not call many mighty in that regard. God uses the weak, and He will glorify Himself in them. Moses did not yet recognize this principle. This would be God's work, the focus would be on God, and what God supplied would always be sufficient for the task. Learning and keeping our place in God's plan is a very hard lesson for us to learn.
In verse 14, God becomes angry at Moses' resistance and his underlying disbelief. God's promise to be with him did not mean that Moses would suddenly become eloquent and fluent. God knows how to use His creatures, and He will use them to His ends. If a man has great resources, his sufficiency makes God unnecessary, and he becomes puffed up. So through Paul, God makes clear that He purposely calls the weak for His purpose.
John W. Ritenbaugh
Prophets and Prophecy (Part 1)