Granted, Jesus was a great man of history. That is a fact. But, He is different from all the other great men of history. How many great men of history rose from the dead, calmed a sea, walked on water, raised others from the dead, healed sickness, and forgave sins? There aren't any others that I know of. Do you know of any? These things make Him more than great. They make Him special and unique.
You are right, Jesus was a great man. But let me ask you. If He were great, would He lie? Of course not. If He were great, would He be insane? No. You see, Jesus said He was God (John 1:1,14; 10:30-33; 20:28; Col. 2:9; Phil. 2:5-8; Heb. 1:8). If He were lying, we shouldn't listen to Him, and we couldn't call Him great. If He were insane, then we shouldn't listen to Him, and again, we couldn't call Him great. If He is great, then He must be telling the truth. And He was great, right?
John 1:1 says, "In the beginning was the Word and the Word was with God and the Word was God." Verse 14 says, "and the Word became flesh..." The Word is Jesus. This means that Jesus is God in flesh. If this is so, then He cannot be merely "a great man of history." He would be far more than that.