John 6:45, "It is written in the prophets, And they shall be all taught of God. Every man therefore that hath heard, and hath learned of the Father, cometh unto me."
The Old Testament prophets prophesied of a new covenant where we would all be taught of God. This is speaking of revelation knowledge that comes from within, through our spirit. Under the Old Covenant, God dealt with the Jews through the outer man. He did this because an Old Covenant man (who was not born again) could not perceive the spiritual truths revealed in the New Covenant by the Spirit (1 Corinthians 2:14). This difference between the old and new covenants is the reason why there had to be two covenants.
The Old Testament man was comparable to a child in his ability to grasp spiritual truth. It is impossible to explain spiritual truth to a young child, and yet, a child must be restrained from submitting to evil. So, the Word of God teaches us to use the "rod of correction" (Proverbs 13:24; 19:18; 22:15). The child may not understand resisting the devil, and yet, when the devil tries to entice him to steal, he will say "No!" because he fears he will get the rod if he does steal. Likewise, Old Testament saints were restrained from sin by a fear of the wrath and punishment of God. This curbed sin, but it also hindered them from receiving the goodness and love of God (1 John 4:18).
Under the New Testament, which Jesus is speaking of in this verse, the punishment for our sins was laid on Him. We no longer serve God out of fear of punishment like once we did when we were "children" under the law, but we serve Him out of love; because He has sent the Spirit of His Son into our hearts, making us His sons. We serve Him because our nature has been changed through the new birth. Today, be taught of God through the indwelling presence of Christ.