Jesus saw Nathanael coming to him: and he saith of him: Behold an Israelite indeed, in whom there is no guile. Nathanael saith to him: Whence knowest thou me? Jesus answered, and said to him: Before that Philip called thee, when thou wast under the fig tree, I saw thee. Nathanael answered him, and said: Rabbi, thou art the Son of God, thou art the King of Israel. Jesus answered, and said to him: Because I said unto thee, I saw thee under the fig tree, thou believest: greater things than these shalt thou see.
And he saith to him: Amen, amen I say to you, you shall see the heaven opened, and the angels of God ascending and descending upon the Son of man. [John 1:47-51]
When Jesus said this to Nathanael, the Israelite and his Israelite companions would have recognized the reference to Genesis. Where else do we hear of "angels of God ascending and descending"?
And when [Jacob] was come to a certain place, and would rest in it after sunset, he took of the stones that lay there, and putting under his head, slept in the same place. And he saw in his sleep a ladder standing upon the earth, and the top thereof touching heaven: the angels also of God ascending and descending by it; And the Lord leaning upon the ladder, saying to him: I am the Lord God of Abraham thy father, and the God of Isaac; the land, wherein thou sleepest, I will give to thee and to thy seed. [Genesis 28:11-13]
Jesus identified Himself with the ladder of Jacob's dream, a ladder "standing upon the earth", and the top "touching heaven". That is an important description, because what it means is the ladder establishes contact between the earth and heaven. Jesus, the Ladder, allows the angels of God to ascend and descend, that tells us that the denizens of heaven may use the Ladder. What about the denizens of earth?
We are told in another place of the contact point between heaven and earth. When God sent Adam and Eve out of the Garden of Eden, He "...placed before the paradise of pleasure Cherubims, and a flaming sword, turning every way, to keep the way of the tree of life." [Gen. 3:24] A flaming sword turning every way guarded the way to the Tree of Life. What was the fruit of the Tree of Life? In a similar way that the Law given to Moses was the fullness of the fruit of the Tree of Knowledge, the fullness of the fruit of the Tree of Life was Jesus Christ. God jealously guarded His plan for Incarnation and Salvation, but He gave clues. "For the Lord God doth nothing without revealing his secret to his servants the prophets." [Amos 3:7] So these two events describe the contact point between heaven and earth, and both refer to the Messiah. If God needed to guard the way to the Tree of Life, then there must have been a place to go in by. Following his vision, Jacob said, "...How terrible is this place! this is no other but the house of God, and the gate of heaven." [Gen 28:17] He refers to the "gate of heaven".
Jesus therefore said to them again: Amen, amen I say to you, I am the door of the sheep. All others, as many as have come, are thieves and robbers: and the sheep heard them not. I am the door. By me, if any man enter in, he shall be saved: and he shall go in, and go out, and shall find pastures. [John 10:7-9]
When Jesus says He is "the door", He means doorway. Often the argument is given by someone who disputes the words of consecration ("This is my body") that "it's a metaphor, like Jesus calling Himself 'a door.' He's not a door". Yes, He is very much a door, referring to doorway. A physical door doesn't allow ingress and egress, it is there to prevent such movement.
To summarize, there must be a physical contact point between Heaven and earth, otherwise we would have no way to ascend the Ladder. We can't "faith" our way up a "spiritual" ladder, we must make contact with the physical body of the Lord. Our means of doing so is in the act of receiving Holy Communion. At the Catholic Mass, Heaven becomes present on earth, because Jesus is physically Present- Body and Blood, Soul and Divinity. This Mass was the purpose of the Incarnation of the Second Person of the Holy Trinity.