Objection 1. It would seem that Christ should not have taught all things openly. For we read that He taught many things to His disciples apart: as is seen clearly in the sermon at the Supper. Wherefore He said: "That which you heard in the ear in the chambers shall be preached on the housetops" [St. Thomas, probably quoting from memory, combines Matthew 10:27 with Luke 12:3]. Therefore He did not teach all things openly.
Objection 2. Further, the depths of wisdom should not be expounded save to the perfect, according to 1 Corinthians 2:6: "We speak wisdom among the perfect." Now Christ's doctrine contained the most profound wisdom. Therefore it should not have been made known to the imperfect crowd.
Objection 3. Further, it comes to the same, to hide the truth, whether by saying nothing or by making use of a language that is difficult to understand. Now Christ, by speaking to the multitudes a language they would not understand, hid from them the truth that He preached; since "without parables He did not speak to them" (Matthew 13:34). In the same way, therefore, He could have hidden it from them by saying nothing at all.
On the contrary, He says Himself (John 18:20): "In secret I have spoken nothing."
I answer that, Anyone's doctrine may be hidden in three ways. First, on the part of the intention of the teacher, who does not wish to make his doctrine known to many, but rather to hide it. And this may happen in two ways--sometimes through envy on the part of the teacher, who desires to excel in his knowledge, wherefore he is unwilling to communicate it to others. But this was not the case with Christ, in whose person the following words are spoken (Wisdom 7:13): "Which I have learned without guile, and communicate without envy, and her riches I hide not." But sometimes this happens through the vileness of the things taught; thus Augustine says on John 16:12: "There are some things so bad that no sort of human modesty can bear them." Wherefore of heretical doctrine it is written (Proverbs 9:17): "Stolen waters are sweeter." Now, Christ's doctrine is "not of error nor of uncleanness" (1 Thessalonians 2:3). Wherefore our Lord says (Mark 4:21): "Doth a candle," i.e. true and pure doctrine, "come in to be put under a bushel?"
Secondly, doctrine is hidden because it is put before few. And thus, again, did Christ teach nothing in secret: for He propounded His entire doctrine either to the whole crowd or to His disciples gathered together. Hence Augustine says on John 18:20: "How can it be said that He speaks in secret when He speaks before so many men?... especially if what He says to few He wishes through them to be made known to many?"
Thirdly, doctrine is hidden, as to the manner in which it is propounded. And thus Christ spoke certain things in secret to the crowds, by employing parables in teaching them spiritual mysteries which they were either unable or unworthy to grasp: and yet it was better for them to be instructed in the knowledge of spiritual things, albeit hidden under the garb of parables, than to be deprived of it altogether. Nevertheless our Lord expounded the open and unveiled truth of these parables to His disciples, so that they might hand it down to others worthy of it; according to 2 Timothy 2:2: "The things which thou hast heard of me by many witnesses, the same command to faithful men, who shall be fit to teach others." This is foreshadowed, Numbers 4, where the sons of Aaron are commanded to wrap up the sacred vessels that were to be carried by the Levites.
Reply to Objection 1. As Hilary says, commenting on the passage quoted, "we do not read that our Lord was wont to preach at night, and expound His doctrine in the dark: but He says this because His speech is darkness to the carnal-minded, and His words are night to the unbeliever. His meaning, therefore, is that whatever He said we also should say in the midst of unbelievers, by openly believing and professing it."
Or, according to Jerome, He speaks comparatively--that is to say, because He was instructing them in Judea, which was a small place compared with the whole world, where Christ's doctrine was to be published by the preaching of the apostles.
Reply to Objection 2. By His doctrine our Lord did not make known all the depths of His wisdom, neither to the multitudes, nor, indeed, to His disciples, to whom He said (John 16:12): "I have yet many things to say to you, but you cannot bear them now." Yet whatever things out of His wisdom He judged it right to make known to others, He expounded, not in secret, but openly; although He was not understood by all. Hence Augustine says on John 18:20: "We must understand this, 'I have spoken openly to the world,' as though our Lord had said, 'Many have heard Me'... and, again, it was not 'openly,' because they did not understand."
Reply to Objection 3. As stated above, our Lord spoke to the multitudes in parables, because they were neither able nor worthy to receive the naked truth, which He revealed to His disciples.
And when it is said that "without parables He did not speak to them," according to Chrysostom (Hom. xlvii in Matth.), we are to understand this of that particular sermon, since on other occasions He said many things to the multitude without parables. Or, as Augustine says (De Qq. Evang., qu. xvii), this means, "not that He spoke nothing literally, but that He scarcely ever spoke without introducing a parable, although He also spoke some things in the literal sense."
Source: Summa Theologica (New Advent)