The angel of Yahweh said to Manoah, Though you detain me, I won't eat of your bread; and if you will make ready a burnt offering, you must offer it to Yahweh. For Manoah didn't know that he was the angel of Yahweh.
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.