Exodus 4:15
And thou shalt speak unto him, and put words in his mouth: and I will be with thy mouth, and with his mouth, and will teach you what ye shall do. And thou shalt speak unto him, and put words in his mouth: and I will be with thy mouth, and with his mouth, and will teach you what ye shall do.
The promise was given at the burning bush near Horeb, the mountain of God, when the time for Israel's deliverance had come. There God revealed Himself as the God of Abraham, Isaac, and Jacob, and called Moses to go forth as a humble shepherd with only a rod in his hand, which God would make the symbol of His power, to bring Israel out of Egypt in a manner to pour contempt on human pride. It belongs to God's persuading of Moses to accept the work of delivering Israel. Moses felt incapable and pleaded not to be given the responsibility, but consented after he was convinced that he was chosen for the task, and his years among the rugged mountains and sheep had taught him humility.
God joined Moses and Aaron together because Moses excelled in wisdom for conducting affairs but did not have ready use of the Egyptian language, while Aaron, his elder brother, used it daily, understood it perfectly, and was eloquent. Moses was to speak to Aaron and put the message and words in his mouth. Aaron was appointed to speak to the people for Moses as his mouth, mouthpiece, and spokesman. Moses was to stand to Aaron in the place of God, remaining responsible for directing Aaron as God's representative.
God promised to be with the mouth of both brothers, to supply the words they were to say, and to teach them what to do, thus providing for Moses' weakness. Moses' calling depended not on his own wisdom or strength but on God's instruction and support. Though self-distrustful, slow of speech, and timid, after accepting the work he trusted God and obeyed wholeheartedly, and God blessed his obedience and strengthened him, making him eloquent, hopeful, and self-possessed for his mission and a channel through which God could work. God would give Moses the words to speak and be with both Moses and Aaron, teaching them what to do.
The verse shows the general truth that God can provide a human mouthpiece and give that person His Spirit for the work He assigns. It is reflected in Aaron's call to join Moses in the mission of leading Israel from Egypt and in his cooperation with Moses in that work.