Manoah said to his wife, We shall surely die, because we have seen God.
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.