"What are the Keys?
Matthew 16
19 And I will give unto you the keys of the kingdom of heaven; and whatsoever you shall bind on earth, shall be bound in heaven; and whatsoever you shall loose on earth shall be loosed in heaven.
*The first thing to consider is what constitutes the keys of the 
kingdom of heaven.
A key is that which unlocks or locks;it is anything by which we gain access to any place, or which enables us to understand any given thing.
Now what is it that opens heaven to mankind, and enables us to understand God?
Evidently the gospel, and nothing else. Paul says that Christ has
2 Timothy 1
10 ...brought life and immortality to light through the gospel.
Immortality stands for all heavenly blessings, as it comprises all. It is the gift of God through Jesus Christ our Lord.
Christ declares himself to be Revelation 3
7 ...he that has the key of David, he that opens, and no man shuts; and shuts, and no man opens.
Isaiah 55
3 Incline your ear, and come unto me; hear, and your soul shall live; and I will make an everlasting covenant with you, even the sure mercies of David.
Thus the gospel opens and shuts.
It seems plain, therefore,that when Christ said,“And I will give unto you the keys of the kingdom of heaven,”he referred to the gospel, which he was about to commit to Peter and his associates."
E.J. Waggoner