Christ Fully Man

Some have mistakenly argued that if Christ is God, He cannot be fully human.  Docetism was "a Christian heresy and one of the earliest Christian sectarian doctrines, affirming that Christ did not have a real or natural body during his life on earth but only an apparent or phantom one."

Among our basic memory verses, there are two key verses that establish the humanity of Jesus.  In John 1:14 it says: "And the Word became flesh, and dwelt among us, and we saw His glory, glory as of the only begotten from the Father, full of grace and truth."  Luke 1:37 says that "nothing shall be impossible with God."  If a barren woman, Elizabeth, could be made to conceive, then the omnipotent God could also cause conception to occur in a woman who had not had relations with a man.  He was the Holy One, but he was also born, just as any other of us starts our life, as an helpless baby (Luke 1:35).   He displayed all the characteristics of a human.  For instance:
He was born (Luke 2:6-7; Gal. 4:4)
He grew in wisdom and stature (Luke 2:52)
He slept (Matt. 8:24)
He thirsted (John 19:28)
He became hungry (Mark 11:12)
He suffered pain (1 Peter 4:1)
He was tempted, but without sinning (Heb. 4:15)
He died (1 Cor. 15:2,3)

The other key memory verse is Gal. 4:4-5, "But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law, so that He might redeem those who were under the Law, that we might receive the adoption as sons."

The reason why Jesus had to be born, and born under the Law, was that He had to fulfill the requirements of the Law, becoming the perfect sacrifice for sin, a man who could die, but as the God-Man, he was able to lay down His life and take it up again, just as it says in John 10:17-19:
" For this reason the Father loves Me, because I lay down My life so that I may take it again. ¹⁸ No one has taken it away from Me, but I lay it down on My own initiative. I have authority to lay it down, and I have authority to take it up again. This commandment I received from My Father.”

As an infinite Person, He could take upon Himself an infinite penalty for all the sin of mankind, for all time, yet do so in a limited time period while on the cross.  As man, He could die, satisfying the penalty for sin, i.e., death (Rom. 6:23).

This concept of being fully God and fully man, united in one Person forever, neither co-mingled nor lacking in either aspect is called the "hypostatic union."  You can read more about it here.