"And He saith unto him, Verily, verily, I say unto you, Hereafter ye shall see heaven open, and the angels of God ascending and descending upon the Son of man" (John 1:51).
In Jacob's dream (Genesis 28:12), angels were ascending and descending on a great ladder between Earth and heaven. Christ promised a future reality in which He Himself would be the ladder to the opened heaven.
But that was to be "hereafter." Until Christ came to die and rise again, heaven was closed, for nothing unclean could enter there and death had "passed upon all men, for that all have sinned" (Romans 5:12). Even those who died in faith could only be "comforted" in "Abraham's bosom" deep "in the heart of the earth" (Luke 16:22,25; Matthew 12:40), because "it is not possible that the blood of bulls and of goats should take away sins" (Hebrews 10:4).
Then Christ died and rose again "to put away sin by the sacrifice of Himself" (Hebrews 9:26). In His spirit, "He also descended first into the lower parts of the earth," and "when He ascended up on high, He led captivity captive" (Ephesians 4:8-9), leading all pre-Calvary saints with Him into heaven. Then was fulfilled the wonderful scene predicted in Psalm 24:7: "Lift up your heads, O ye gates; and be ye lift up, ye everlasting doors; and the King of glory shall come in."
Since that wonderful day when Christ ascended back to heaven "to be absent from the body" is "to be present with the Lord" (II Corinthians 5:8). Heaven someday will even receive our resurrected bodies. John prophesied it this way: "I looked, and, behold, a door was opened in heaven: and the first voice . . . said, Come up hither, and I will shew thee things which must be hereafter" (Revelation 4:1). Heaven thenceforth will be open eternally to all the redeemed. "The gates of it shall not be shut at all by day: for there shall be no night there" (Revelation 21:25). HMM