27. That is, the Messenger (peace be upon him) by himself is no knower of the unseen, but when Allah chooses him to perform the mission of Prophethood, He grants him the knowledge of those of the unseen truths which He is pleased to grant.
28. Guards: Angels, that is, when Allah sends down the knowledge of the unseen realities to the Messenger (peace be upon him) by revelation, He appoints angels on every side to safeguard it so that the knowledge reaches the Messenger in a safe condition, free from any kind of adulteration. This is the same thing which has been expressed in verses 8-9 above, saying: After the appointment of the Messenger (peace be upon him) the jinn found that all the doors to the heavens had been closed, and they noticed that strict security measures had been adopted because of which no room had been left for them to eavesdrop.