Pidyon Haben and The Son of God.

The Laws of Pidyon Haben(redemption of the firstborn, Numbers 18:15) according to Jewish tradition began after the Egel haZahav(golden calf) incident. Before that the first born male of every tribe was to be set aside for Mishkan/Beis HaMikdash(Tabernacle/Temple) service(see Exodus 13:1-2, Exodus 24:5 with Rashi). The tribe of Levi was the only one that did not participate in the sin of the Egel haZahav so the whole tribe was dedicated to Mishkan/Beis HaMikdash service. So it seems that Pidyon Haben was a release, mandated by God, that every other tribe had to do regardless of whether or not the father of that child wanted his firstborn to become a priest. Christ not being of the tribe of Levi would have to have a Father that was not of any of the tribes that sinned with the golden calf and thus not be subject to the laws of Pidyon Haben making him eligible for the Priesthood.