There is no way our Lord Jesus Christ had a human form before His assumption of one exactly 2011 years ago. God is absolutely immutable, perfect, and cannot be undone, but a human body is changing, imperfect, and may pass away. How can we possibly believe that it is in God’s very essence to have a human body, given His infinite and eternal character? I see no way of reconciling this notion with the perfection of our Lord.
Regarding it right for our salvation, He took the human form and nature and united His perfect divine essence to it.

By taking the mutable and destructible and imparting undying life to it, He elevated humanity beyond our understanding. The Lord’s human nativity occurred only once, and is not a thing in eternity. Perhaps we can say that Christ is the “Lamb slain from the beginning” in the same way that we can say God remains the “Creator from the beginning”; that is, Christ upholds His plan of salvation-by-union-and-death across all generations, just as the Father is continually creating and “holding up” the Universe by His essence.
One of the great mysteries of the Universe is the fact that God now has a human body: forever. It is something that will never die away, because He has united humanity so perfectly to Himself that the very essence of “human” has become greater than before. Christ our Lord is literally between Heaven and Earth, now united to a physical body FOR ALL TIME, and even beyond time when He shall transfigure us all, if we become saints.
We profess the Lamb slain from the beginning because God began this Universe solely with the intention of uniting sentient beings to Himself by Jesus Christ - through the means He had set down to be followed, from all eternity. It need not have been the Cross, for all we know, but any manner of lowly death. He is slain from the beginning because He decided that is how we will be united to Him. Men are raised up with Him, and when manhood was immolated on the Cross, all mankind was burned away of its impurities.
How glorious is our God, Lord of majesty!