The Calling of St James to the Apostleship appears to be Mantegna's design, partially carried out by Pizzolo; the subjects of St James baptizing, his appearing before the judge, and going to execution, and most of the legend of St Christopher, are entirely by Mantegna.
In this is proclaimed the doctrine of the one church founded upon the apostle Peter, whose "tangible bond is her one united episcopate, an apostleship universal yet only one - the authority of every bishop perfect in itself and independent, yet not forming with all the others a mere agglomeration of powers, but being a tenure upon a totality like that of a shareholder in some joint property."
That they had a large measure of authority of course goes without saying, but it depended always upon their brethren's recognition of their possession of the divine gift of apostleship, and the right of Churches or individuals to test their claims and to refuse to listen to them if they did not vindicate their divine call was everywhere recognized.
Apostleship, prophecy and teaching were only functions, whose frequent or regular exercise by one or another, under the inspiration of the Spirit, led his brethern to call him an apostle, prophet or teacher.
Now, however, emerges a new apostleship, that to the Gentiles; and with the change of mission goes also some change in the type of missionary or apostle.