iirc, the prophecy said that the one leading the revolution wasn't yet born, hence he had to rule Shanks out obviously. It has little to do with potential, since Roger acknowledged they were too early too. Because it's not about potential as in battle power, but just being the Messiah that is reborn, who was worshipped as a god by various nations, including Elbaf. Obviously Shanks cannot meet that criteria. On the other hand, Roger didn't have "faith in his unborn son", it was just wishful thinking that his son will be the one the prophecy talks about.