Whilst it is true that “God was manifest in flesh”, what is important to consider in the context is that the the Lord's life of devotion was a demonstration of the life which God desires to see in each of us. The teaching of John 1:18 is stating the fact that God was seen in perfection in the Son. Here it is His devotion that is emphasised.
“Justified in the Spirit” may indicate that the life was vindicated by the Holy Spirit. This is seen in the way in which the Spirit led Him into the wilderness (Luke 4:1), demonstrating absolute confidence in His ability to overcome the devil. It is seen in His resurrection, the ultimate declaration both of His Sonship (Rom 1:4) and of the value of His death (Rom 4:25). It may also indicate that the spirit in which His life was lived has been manifestly vindicated (Phil 2:9-11). “Seen of angels” reminds us that angels had seen a world created in perfection, but corrupted by sin. Now they see One coming into the world to redeem.
“Preached unto the Gentiles” reminds us that, where Israel failed to bear witness as God intended, Christ perfectly reveals God to all.
“Believed on in the world” is the acceptance by men of His completed work.
“Received up in glory” is His ultimate vindication. He left heaven as Son – He returned not just as Son, but as perfect man and Saviour of the world. May we have grace to follow His steps.