In regard to the prophets among the Jews, some of them were wise men before they became divinely inspired prophets, while others became wise by the illumination which their minds received when divinely inspired. They were selected by Divine Providence to receive the Divine Spirit, and to be the depositaries of His holy oracles, on the ground of their leading a life of almost unapproachable excellence, intrepid, noble, unmoved by danger or death. For reason teaches that such ought to be the character of the prophets of the Most High, in comparison with which the firmness of Antisthenes, Crates, and Diogenes will seem but as child's play.
It was therefore for their firm adherence to truth, and their faithfulness in the reproof of the wicked, that “they were stoned; they were sawn asunder, were tempted, were slain with the sword; they wandered about in sheepskins and goatskins; being destitute, afflicted, tormented; they wandered in deserts and in mountains, and in dens and caves of the earth, of whom the world was not worthy:” for they looked always to God and to His blessings, which, being invisible, and not to be perceived by the senses, are eternal.
We have the history of the life of each of the prophets; but it will be enough at present to direct attention to the life of Moses, whose prophecies are contained in the law; to that of Jeremiah, as it is given in the book which bears his name; to that of Isaiah, who with unexampled austerity walked naked and barefooted for the space of three years. Read and consider the severe life of those children, Daniel and his companions, how they abstained from flesh, and lived on water and pulse. Or if you will go back to more remote times, think of the life of Noah, who prophesied; and of Isaac, who gave his son a prophetic blessing; or of Jacob, who addressed each of his twelve sons, beginning with “Come, that I may tell you what shall befall you in the last days.” These, and a multitude of others, prophesying on behalf of God, foretold events relating to Jesus Christ.
We therefore for this reason set at nought the oracles of the Pythian priestess, or those delivered at Dodona, at Clarus, at Branchidæ, at the temple of Jupiter Ammon, or by a multitude of other so-called prophets; while we regard with reverent awe the Jewish prophets: for we see that the noble, earnest, and devout lives of these men were worthy of the inspiration of the Divine Spirit, whose wonderful effects were widely different from the divination of demons.
Source: Against Celsus (New Advent)