3 “And there was a man of the Pharisees, Nicodemus by name, a ruler of the Jews: the same came to Him by night, and said unto Him, Rabbi (you already know that Master is called Rabbi), we know that You are a teacher come from God; for no man can do these signs which You do, except God be with him.” This Nicodemus, then, was of those who had believed in His name, as they saw the signs and prodigies which He did. For this is what he said above: “Now, when He was in Jerusalem at the passover on the feast-day, many believed in His name.”
Why did they believe? He goes on to say, “Seeing His signs which He did.” And what says he of Nicodemus? “There was a ruler of the Jews, Nicodemus by name the same came to Him by night, and says to Him, Rabbi, we know that You are a teacher come from God.” Therefore this man also had believed in His name. And why had he believed? He goes on, “For no man can do these signs which You do, unless God be with him.” If, therefore, Nicodemus was of those who had believed in His name, let us now consider, in the case of this Nicodemus, why Jesus did not trust Himself to them.
“Jesus answered and said unto him, Verily, verily, I say unto you, Unless a man be born again, he cannot see the kingdom of God.” Therefore to them who have been born again does Jesus trust Himself. Behold, those men had believed on Him, and yet Jesus trusted not Himself to them. Such are all catechumens: already they believe in the name of Christ, but Jesus does not trust Himself to them. Give good heed, my beloved, and understand. If we say to a catechumen, Do you believe in Christ, he answers, I believe, and signs himself; already he bears the cross of Christ on his forehead, and is not ashamed of the cross of his Lord.
Behold, he has believed in His name. Let us ask him, Do you eat the flesh of the Son of man, and drink the blood of the Son of man? He knows not what we say, because Jesus has not trusted Himself to him.
Source: Tractates on the Gospel of John (New Advent)