What has to be kept secret? Actually, there is nothing in Vajrayana that is shameful or inappropriate and cannot be told. Rather, it is the profound and uncommon view of Vajrayana, such as the equality of defilement (klesa) and wisdom (bodhi), samsara and nirvana, sentient beings and the enlightened, etc., that needs to be kept confidential. If not, many people may end up with a mistaken view of Vajrayana because they cannot comprehend the concepts.
~Depicted from GATEWAY TO VAJRAYANA PATH - A Compendium of the Vajrayana