The calling of Moses recorded in the Scriptures leaves no doubt that he was not a man without sin or weakness. By the time we get to Exodus 4 we are told of his third, fourth, and fifth objections (Exodus 3:11, Exodus 3:13; Exodus 4:1, Exodus 4:10, Exodus 4:13).
It would appear that the man who was described as mighty in his words and deeds
(Acts 7:22), was afraid that he had grown rusty in his ability to communicate in Egyptian.
The problem was that Moses was working with human expectations concerning how the job should be done, which gave a very important role to the speaker.
The Lord teaches us that the instrument he uses is not that important, and even though Moses might find it embarrassing to be so under-qualified for the task, the Lord had chosen Moses as his instrument so that it would be very clear that it was the Lord himself who was delivering his people Israel from slavery.
The Lord’s Name is glorified even more when he uses weak and unlikely instruments to fulfil his plans.
10 But Moses said to the LORD, “Oh, my Lord, I am not eloquent, either in the past or since you have spoken to your servant, but I am slow of speech and of tongue.”