For it happened, when the flame went up toward the sky from off the altar, that the angel of Yahweh ascended in the flame of the altar: and Manoah and his wife looked on; and they fell on their faces to the ground.
The same Hebrew word translated "wonderful" appears in Isaiah 9:6 as a title of Messiah. The idea here is that the Angel said Manoah and his wife could not fully comprehend the significance of who He was (cf. Exodus 3:13-14; Isaiah 9:6; Psalms 139:6). Though we can apprehend God to some extent, we cannot fully comprehend Him. Sinful mortals cannot fully appreciate all there is to know about God, even with the aid of the revelation He has given us.