And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams: In the last days - The time of the Messiah; and so the phrase was understood among the Jews.
I will pour out of my Spirit upon all flesh - Rabbi Tanchum says, "When Moses laid his hands upon Joshua, the holy blessed God said, In the time of the old text, each individual prophet prophesied; but, in the times of the Messiah, all the Israelites shall be prophets." And this they build on the prophecy quoted in this place by Peter.
Your sons and your daughters shall prophesy - The word prophesy is not to be understood here as implying the knowledge and discovery of future events; but signifies to teach and proclaim the great truths of God, especially those which concerned redemption by Jesus Christ.
Your young men shall see visions, etc. - These were two of the various ways in which God revealed himself under the Old Testament. Sometimes he revealed himself by a symbol, which was a sufficient proof of the Divine presence: fire was the most ordinary, as it was the most expressive, symbol. Thus he appeared to Moses on Mount Horeb, and afterwards at Sinai; to Abraham, Genesis 15:1-21; to Elijah, 1 Kings 19:11, 1 Kings 19:12. At other times he revealed himself by angelic ministry: this was frequent, especially in the days of the patriarchs, of which we find many instances in the book of Genesis.
By dreams he discovered his will in numerous instances: see the remarkable case of Joseph, Genesis 37:5, Genesis 37:9; of Jacob, Genesis 28:1, etc.; Genesis 46:2, etc.; of Pharaoh, Genesis 41:1-7; of Nebuchadnezzar, Daniel 4:10-17. For the different ways in which God communicated the knowledge of his will to mankind, see the note on Genesis 15:1.