well i dont knoe about the "keys to heaven" but the NT refres to the "keys of hell".
rev. 1:17-19
"And when I saw him, I fell at his feet as dead. And he laid his right hand upon me, saying unto me, Fear not; I am the first and the last: I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death. Write the things which thou hast seen, and the things which are, and the things which shall be hereafter;
--with all the mysticism aside basicly after Christ's death, He went to "hell" (hades 'abode of the dead') *(1pet 3:18-20, acts 2:27-31)
--and "left" 'hell' *(abode of the dead) as eph 4:8-10 states. "Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men. He that descended is the same also that ascended up far above all heavens, that he might fill all things.)
the "leaving captivity captive" thing would be like saying "leving that which 'holds on' 'grasping' " also in similarity when Christ says "the gates of hell will not prevail."
Gates are a defensive structure, and its like saying "hell cant keep us in"
the only "keys of heaven" i found was in matt16:19 "And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven."
but thats speaking of the gospel itself *(whatever it is) as part of the last part of the verse.