2, 3. the angel of the Lord appeared unto him in a flame of fire--It is common in Scripture to represent the elements and operations of nature, as winds, fires, earthquakes, pestilence, everything enlisted in executing the divine will, as the "angels" or messengers of God. But in such cases God Himself is considered as really, though invisibly, present. Here the preternatural fire may be primarily meant by the expression "angel of the Lord"; but it is clear that under this symbol, the Divine Being was present, whose name is given (Ex 3:4, 6), and elsewhere called the angel of the covenant, Jehovah-Jesus.
      out of the midst of a bush--the wild acacia or thorn, with which that desert abounds, and which is generally dry and brittle, so much so, that at certain seasons, a spark might kindle a district far and wide into a blaze. A fire, therefore, being in the midst of such a desert bush was a "great sight." It is generally supposed to have been emblematic of the Israelites' condition in Egypt--oppressed by a grinding servitude and a bloody persecution, and yet, in spite of the cruel policy that was bent on annihilating them, they continued as numerous and thriving as ever. The reason was "God was in the midst of them." The symbol may also represent the present state of the Jews, as well as of the Church generally in the world.

JFB.