Garga Muni continued to say, “My dear King of Vraja, in His previous births this child many times protected righteous persons from the hands of rogues and thieves whenever there was political disruption. Your child is so powerful that anyone who will become devotee of your boy will never be troubled by enemies. Just as demigods are always protected by Lord Vishnu, the devotees of your child will always be protected by NÄrÄyana, the Supreme Personality of Godhead. Therefore I would advise that you protect Him very carefully so that He may grow without disturbance.” In other words, Garga Muni informed Nanda Maharaj that because Nanda was a great devotee of Lord Narayana, the Lord has given Nanda a son equal to Himself. At the same time, Garga Muni indicated that this son would be disturbed by many demons that’s why Nanda should protect Him. In this way Garga Muni convinced Nanda Maharaj that Narayana Himself has become his son!
(except from ‘Krishna Book’ chapter-8 by A.C Bhaktivedanta Swami Srila Prabhupad)