What is meant by ‘God’ in this phrase is the god head, His essential divinity, because this god head is invisible. And in regard to this divinity since God is Spirit, He cannot be seen by our earthly eyes which can only perceive material things.
This is why God always appeared in the form of a human being or in the shape of an angel when it was intended for human beings to see Him. And finally He appeared in the body when we saw Him in His Son Jesus Christ who said: “Anyone who has seen Me has seen the Father. ” (John 14:9)
Thus after John the Baptist said “No-one has ever seen God”, he went on to say: “The Only Begotten Son, who is in the Father’s bosom, has made Him known. ” (John 1:18), which means that He declared the Father.
All those who try to depict or have ever tried to portray the Father in a visible form, have got it wrong, and this very verse proves them wrong, for example those who portray the Father in icons of the baptism of Christ, when God said: “This is My Son, whom I love; with Him I am well pleased.”" (Matt. 3:17), when no-one actually saw the Father.
All the while that we are in this physical body, there is a mist like a veil preventing us from seeing God, we can only see “a poor reflection as in a mirror. “ as St Paul put it (1 Cor. 13:12). But when we put off this earthly body, we will be clothed in a shining, spiritual body that can see what no eye has ever seen. And it is then that we shall see God.