Jesus taught us who it was that sent Him, in the words, “None knows the Father but the Son;” and in these, “No man has seen God at any time; the only-begotten Son, who is in the bosom of the Father, He has declared Him.” He, treating of Deity, stated to His true disciples the doctrine regarding God; and we, discovering traces of such teaching in the Scripture narratives, take occasion from such to aid our theological conceptions, hearing it declared in one passage, that “God is light, and in Him there is no darkness at all;” and in another, “God is a Spirit, and they that worship Him must worship Him in spirit and in truth.” But the purposes for which the Father sent Him are innumerable; and these any one may ascertain who chooses, partly from the prophets who prophesied of Him, and partly from the narratives of the evangelists. And not a few things also will he learn from the apostles, and especially from Paul. Moreover, those who are pious He leads to the light, and those who sin He will punish—a circumstance which Celsus not observing, has represented Him “as one who will lead the pious to the light, and who will have mercy on others, whether they sin or repent.”
Source: Against Celsus (New Advent)