3 It was in answer to Nicodemus, who was of them that had believed on Jesus, that it was said, And Jesus did not trust Himself to them. To certain men, indeed, He did not trust Himself, though they had already believed on Him. Thus it is written, “Many believed in His name, seeing the signs which He did. But Jesus did not trust Himself to them. For He needed not that any should testify of man; for Himself knew what was in man.” Behold, they already believed on Jesus, and yet Jesus did not trust Himself to them.
Why? Because they were not yet born again of water and of the Spirit. From this have we ex horted and do exhort our brethren the catechumens. For if you ask them, they have already believed in Jesus; but because they have not yet received His flesh and blood, Jesus has not yet trusted Himself to them. What must they do that Jesus may trust Himself to them? They must be born again of water and of the Spirit; the Church that is in travail with them must bring them forth. They have been conceived; they must be brought forth to the light: they have breasts to be nourished at; let them not fear lest, being born, they may be smothered; let them not depart from the mother's breasts.
Source: Tractates on the Gospel of John (New Advent)