In exploring the life of Jesus Christ as fully God and fully man it is comparatively easier to state that, of course, He is all that God is, and that is, well, because we apply our limited knowledge of God and that's that, even when we don't really fully know God. But on the side "fully man" we need to look at our first example, Adam BEFORE he sinned. Can we define what fully man is looking at mankind from a best case scenario of what we think man is without sin? Just what exactly is man?
We know that Adam was made in the image and likeness of God (Genesis 1:26 - 27) But thinking of being fully man we see Christ being called the last Adam.
But now is Christ risen from the dead, and was made the first fruits of them that slept. For since by man came death, by man came also the resurrection of the dead. For as in Adam all die, even so in Christ shall all be made alive,
It is sown a natural body, and is raised a spiritual body: there is a natural body, and there is a spiritual body. As it is also written, The first man Adam was made a living soul: and the last Adam was made a quickening Spirit. Howbeit that was not first which is spiritual: but that which is natural, and afterward that which is spiritual. The first man is of the earth, earthly: the second man is the Lord from heaven.
1 Corinthians 15:20 - 22, 44 - 47 GNV
There is something definite in the characteristics of the life of Adam after he was created and before he sinned. Then there was something different in the life of Jesus after His birth and before His crucifixion. We are told this man, Jesus Christ, was in all points tempted as we are. And there was so much more in his statement when Pilate said, Behold, the man!
Why all these questions about man for this psalm? There are several psalms that tell of man, the children of Israel in particular many different ways, but often specifically about God's hearing their cry out of Egypt, the plagues He brought, the deliverance wrought of His people from Egypt, through the Red Sea, and the wanderings in the wilderness. This psalm is one of them. Nearly every psalm including this one deals with the struggles and responses as men.
We might consider "how pure" it is to worship, to sing joyfully unto God our strength. Both the man who has never sinned and the one who knows they have been redeemed agree in thie praise written in these first lines of this psalm. What comes next is that time of proving, which was demonstrated the hearts of the children of Israel often in the wilderness. The heart of the one who never sinned feels the burden as man reading this psalm much like the groaning because of the unbelief at the tomb of Lazarus before he was raised. But the redeemed man has felt and remembers the weight, the burden that unbelief brings, that unfaithfulness draws away from the Lord even as Peter who went out and wept bitterly, yet knew the goodness of restoration. Through that all then being reminded of those who failed to grasp the significance of what the Lord had done in plaguing the Egyptians, in opening the Red Sea for them and then closing it upon their enemies, in caring for them and leading them those forty years in the wilderness, in giving them the land promised to Abraham. It was a weight to both the one who had not sinned and those redeemed from that sin felt within them, still recognized differently. And though recognized differently the condition and need of those who walk away from the Lord, who reject following Him is both the same. When they pray they both cry out the heart of God:
Oh that my people had hearkened unto me, and Israel had walked in my ways. I would soon have humbled their enemies, and turned mine hand against their adversaries. The haters of the Lord should have been subiect unto him, and their time should have endured for ever. And God would have fed them with the fat of wheat, and with honey out of the rock would I have sufficed thee.
Psalms 81:13 - 16 GNV