# have never been eloquent

"have never been an excellent speaker". Although Moses believes in Yahweh, he does not trust in him. This is because Moses lacks understanding. Moses tries to believe the things he is asked to do are done by his own power. Yahweh is trying to get Moses to trust that these are Yahweh’s doing. 

# I am slow of speech and slow of tongue

Moses uses these phrases to emphasize that he is not a good speaker. 

# slow of tongue

"unable to speak well"