Exodus 4:10 And Moses said unto the Lord, O my Lord, I am not eloquent, neither heretofore, nor since thou hast spoken unto thy servant: but I am slow of speech, and of a slow tongue.
God has just told Moses that he is to go and tell Pharaoh to let the nation of Israel go. Moses argues with God about how no one will believe him or hear his voice and God gave Moses two miracles Moses could do; turn the rod into a snake and cause his hand to be leprous. After God gave Moses these two signs, Moses next excuse was that he was not eloquent of speech and as such, he needed an assistant.
God told Moses that HE made the mouth and that Moses was to go and God would teach him what to say. However, Moses was reluctant and he insisted that God give him an assistant. Moses simply did not have the faith to trust that God could use his weakness. God gave Moses Aaron as an assistant. It was Aaron who led the people into worshiping the golden calf. God�s perfect plan was to use Moses� weakness so that God would be glorified and receive all the glory. Moses had received a great calling and while he obeyed, he hampered his ministry because he did not have the faith to trust God to work through his weaknesses.