The protagonist in Michael Moorcock's novel who travels from the future to the time of Christ, Glogauer, instead, meets a profoundly retarded child of Mary who is, in Moorcock's account, most definitely NOT the Son of God. Glogauer then assumes the personae of Jesus of Nazareth, based on his recollection and knowledge of the accounts in the Gospels of the New Testament, culminating in his crucifixion to fulfill those accounts which shaped history to the moment in the future in which he journeyed into the past to complete the story.
Perhaps the most simultaneously unsettling and reassuring aspect of Behold the Man is not the death of someone else in place of the Son of God but its emphasis and reaffirmation of the importance in the belief that He lived at all. For you today for whom this is, an Ecce Homo experience, my sincere congratulations tinged with perhaps a little jealousy and envy.
Not everyone has the comfort of your beliefs and the reassurance of your faith. Some may not wish to have it while others who once did realize again the distance traveled from then to now involved a bridge of faith that, once abandoned, has been destroyed and which can very possibly never be rebuilt.
As even Mark reported, help for one's unbelief is not easily achieved and perhaps the realization that such assistance can only be given and never earned is part of why pride becomes the greater of the sins especially for those with so little reason to be proud. Perhaps it's doubt that creates the whisper of vulnerability in an armor of faith which then allows a wanderer to know the path but who refuses to walk it again. Sometimes it's the belief, and sometimes, the believer.