Jesus gave Peter the keys to open the doors of this kingdom.  Those who believe that the kingdom has not come believe Jesus gave Peter some keys that were of no value to Peter at all.  Keys open doors.  If the kingdom did not come, there were no doors for Peter to open with those keys.  Jesus gift to Peter would have been a vanity.  But Peter did open the doors to the kingdom on the day of Pentecost.  One of the keys that opened the door to the kingdom was of humility–a spiritual key.

And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.  Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven  (Matt. 18:3-4).

God opposes the proud, but gives grace to the humble (1 Pet. 5:5).  Those he opposes are not able to enter into the kingdom.

The question is, who are the spiritual children, and how did they get to become spiritual children?  Jesus showed Nicodemas how to enter this kingdom.  First Jesus said “Except a man be born again, he cannot see the kingdom of God” (John 3:3).  We noted that the kingdom can not even be seen without a spiritual birth. How can we enter it?  “Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God” (John 3:5).  Many different interpretations have been given to this vision.  They cannot all be correct.  Jesus explained that this birth is not a physical birth but a spiritual birth. (John 3:5-8).  Generally speaking, a physical birth puts a person into the physical nation in which he is born.  It is understandable that a spiritual birth would put a person into God’s spiritual kingdom.  This sounds easy enough.  Why would we need to strive to enter it?  The reason is that there are other requirements to get into the kingdom besides seeking.  “And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven” (Matt 18:3).  Not even these apostles were able to enter the kingdom of heaven unless they were converted and became as little children.  How were they to become ‘as little children?’  The next verse tells us: “Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven” (Matt. 18:4).  Humility is not easy to gain!  Humility is one of the things that men must ‘strive’ for, if they are going to enter the kingdom.  Entering the kingdom of God is to be born into it to, and thus to become alive to God by a spiritual birth.  Those who are alive to God are spiritual children.