Then Manoah entreated Yahweh, and said, Oh, Lord, please let the man of God whom you did send come again to us, and teach us what we shall do to the child who shall be born.
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.