Perhaps the most compelling evidence in favor of the Shroud's authenticity is the evidence surrounding what is known as the "four-finger" phenomenon on the image.
While most artwork from the Middle Ages depicting the Crucifixion shows Christ nailed to a cross with nails driven through his hands, the image on the Shroud does not show this. Instead, it clearly shows nail wounds in the wrists, which is exactly how a person would have been nailed to a cross -- because nails driven through the middle of the hand would not support the weight of a human body without tearing through the hand. Point #1: If someone in the 12th century was intent on making a realistic forgery of Christ's burial shroud, then why would he depict the crucifixion in a manner that did not match the prevailing view of how the crucifixion occurred?
More importantly, the hands shown on the image appear to only have four fingers -- leading to speculation that perhaps the person whose image was on the Shroud had his thumbs cut off before "burial." The reality is that the image is anatomically correct, because driving a nail through the wrist between the two bones of the forearm (the radius and the ulna) damages one of the key nerves in the wrist and produces a reflexive reaction in which the thumb is drawn across the palm in such a way that it is not visible from the back of the hand.
Point #2: I find it extremely unlikely that a forger in the Middle Ages would have known such minute detail about human anatomy that he would have been able to replicate the results of this reflexive action.
Point #2: I find it extremely unlikely that a forger in the Middle Ages would have known such minute detail about human anatomy that he would have been able to replicate the results of this reflexive action.
The answer to both of these is the same. He would not have to have known or even thought about these, assuming a real crucified body was used to make the image. All the analysis of the image points conclusively to the fact that the person on the Shroud was crucified. If the forger (assuming there was one) used a crucified body to make the image, and knew how the Romans had done it, then he wouldn't have had to have any intimate medical knowledge about what it does and nerve reactions, etc.
The iconography issue is a legitimate one, though.
That leaves the question of the image formation itself. There is only so long that you can support the theory that a medieval forger can fool modern science. At some point that theory becomes more untenable than the authenticity theory. It seems to me that day is approaching.