Christ ……
prophecy
“Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.”
Isaiah 7:14 KJV
Fulfilment
“And she shall bring forth a son, and thou shalt call his name JESUS: for he shall save his people from their sins. Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, Behold, a virgin shall be with child, and shall bring forth a son, And they shall call his name Emmanuel, which being interpreted is, God with us.”
Matthew 1:21-23 KJV
There are lots of those one was this one
“Surely he hath borne our griefs, and carried our sorrows: yet we did esteem him stricken, smitten of God, and afflicted. But he was wounded for our transgressions, he was bruised for our iniquities: the chastisement of our peace was upon him; and with his stripes we are healed. All we like sheep have gone astray; we have turned every one to his own way; and the LORD hath laid on him the iniquity of us all. He was taken from prison and from judgment: and who shall declare his generation? for he was cut off out of the land of the living: for the transgression of my people was he stricken. And he made his grave with the wicked, and with the rich in his death; because he had done no violence, neither was any deceit in his mouth. Yet it pleased the LORD to bruise him; he hath put him to grief: when thou shalt make his soul an offering for sin, he shall see his seed, he shall prolong his days, and the pleasure of the LORD shall prosper in his hand.”
Isaiah 53:4-6, 8-10 KJV
Another
“The people that walked in darkness have seen a great light: they that dwell in the land of the shadow of death, upon them hath the light shined. …..For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace. Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.”
Isaiah 9:2, 6-7 KJV
Another
“My God, my God, why hast thou forsaken me? Why art thou so far from helping me, and from the words of my roaring? All they that see me laugh me to scorn: They shoot out the lip, they shake the head, saying, He trusted on the LORD that he would deliver him: Let him deliver him, seeing he delighted in him. For dogs have compassed me: The assembly of the wicked have inclosed me: They pierced my hands and my feet. I may tell all my bones: They look and stare upon me. They part my garments among them, And cast lots upon my vesture.”
Psalm 22:1, 7-8, 16-18 KJV
The point being when we say Jesus “ the Christ “ or jesus “ the messiah “ we’re acknowledging that he is the one God had been promising who would come the redeemer and holy one of Israel and the king of kings and lord of lords
When they are talking about Jesus being Christ they’re arguing that he is the one the ot was about who was to come …….