5 Certainly in the formation of the clergy and the sacerdotal ministry, it is reasonable that regard should be had to the varied conditions of the times. Therefore we are far from rejecting the idea of such changes as would render the work of the clergy still more efficacious in the society in which they live, and it is for that reason that it has seemed necessary to us to promote among them a more solid and finished culture, and to open a still wider field to their ministry; but every other innovation which could in any way prejudice what is essential to the priest must be regarded as altogether blameworthy. The priest is above all constituted master, physician and shepherd of souls, and a guide to an end not enclosed within the bounds of this present life. Now he can never fully correspond if he is not well versed in the science of divine and sacred things, if he is not furnished with that piety which makes a man of God; and if he does not take every care to render his teachings valuable by the efficacy of his example, conformably to the admonition given to the sacred pastor by the Prince of the Apostles: "Forma facti gregis ex animo."(7) For those who watch the times and the changeable condition of society, these are the right and the greatest gifts that could shine in the Catholic priest, together with the principles of faith; every other quality natural and human would certainly be commendable, but would not have with regard to the sacerdotal office anything but secondary and relative importance. If, therefore, it is reasonable and just that the clergy should accommodate themselves as far as is permitted to the needs of the present age, it is still more necessary that the present depravity of the century should not be yielded to, but strongly resisted; and this while corresponding naturally to the high end of the priesthood, will also render their ministry still more fruitful by increasing its dignity, and therefore gaining it respect. It is seen everywhere how the spirit of naturalism tends to penetrate every part of the social body, even the most healthy; a spirit which fills the minds with pride and causes them to rebel against every authority; depraves the heart and turns it after the desire of earthly goods, neglecting those eternal.
Source: Fin dal Principio (Vatican.va)