And I have not yet spoken of the observance of all that is written in the Gospels, each one of which contains much doctrine difficult to be understood, not merely by the multitude, but even by certain of the more intelligent, including a very profound explanation of the parables which Jesus delivered to “those without,” while reserving the exhibition of their full meaning for those who had passed beyond the stage of exoteric teaching, and who came to Him privately in the house.
And when he comes to understand it, he will admire the reason why some are said to be “without,” and others “in the house.” And again, who would not be filled with astonishment that is able to comprehend the movements of Jesus; ascending at one time a mountain for the purpose of delivering certain discourses, or of performing certain miracles, or for His own transfiguration, and descending again to heal the sick and those who were unable to follow Him whither His disciples went?
But it is not the appropriate time to describe at present the truly venerable and divine contents of the Gospels, or the mind of Christ— that is, the wisdom and the word— contained in the writings of Paul. But what we have said is sufficient by way of answer to the unphilosophic sneers of Celsus, in comparing the inner mysteries of the Church of God to the cats, and apes, and crocodiles, and goats, and dogs of Egypt.
Source: Against Celsus (New Advent)