The Ascension is the ascent of Jesus Christ into heaven on the fortieth day after his resurrection. The concept owes itself largely to the author of Luke’s gospel, and in particular the opening section of Luke’s Acts of the Apostles, 

“After he [Jesus] said this, he was taken up before their very eyes, and a cloud hid him from their sight. They were looking intently up into the sky as he was going, when suddenly two men dressed in white stood beside them. “Men of Galilee,” they said, “why do you stand here looking into the sky? This same Jesus, who has been taken from you into heaven, will come back in the same way you have seen him go into heaven” (1:9–11, emphasis added).

The gospels have little or no chronological separation between resurrection and ascension. Matthew does not narrate an ascension as such and gives no weight to chronological considerations in his retelling of the final episodes of his gospel (28:16–20). The Gospel of Luke is content to leave the impression that Christ “was taken up into heaven” on the day of the resurrection itself,

“When he had led them out to the vicinity of Bethany, he lifted up his hands and blessed them. While he was blessing them, he left them and was taken up into heaven. Then they worshiped him and returned to Jerusalem with great joy. And they stayed continually at the temple, praising God” (24:50–53, emphasis).

John’s gospel similarly speaks of an ascension happening on the day of the resurrection (20:17), although he also includes an appearance a week later (20:26–29).

Elsewhere in the New Testament, different imagery is used and implies a single movement of resurrection-exaltation. This is apparent in the disciple Peter’s speech in the Acts: “[T]his Jesus God raised up, and of that all of us are witnesses. Being therefore exalted at the right hand of God…” (2:32–33). The Apostle Paul’s (c. 4 BCE–c. 62–64 CE) Philippian hymn notably refers to Christ as one who “became obedient to death, even death on a cross. Therefore God also highly exalted him…” (2:8–9). 

The imagery of the letter Hebrews is one of an entry into the heavenly sanctuary as a priest bearing the sacrificial blood (his own): “[W]hen he had made purification for sins, he sat down at the right hand of the Majesty on high” (1:3).

Why is it only Acts that provides a clear distinction and separation of forty days between the resurrection and ascension of Jesus? Luke intended to mark a definite and indisputable end to the sequence of resurrection appearances, and he did so by stressing the visibility of Jesus’ final departure before witnesses. He describes the resurrection appearances as solid or corporeal, even though the risen body could pass through doors. 

The detail in Acts 1:4 that “while he was staying/eating with them” could suggest lengthy periods of Jesus’ visible encounter with his disciples, although the parallel episodes in Luke’s gospel suggest that the appearances were of relatively short duration (24:31, 51).

Although there is little doubt that Jesus was seen by various people after his death, the sequence of seeings ceased at some point, as Paul agrees (1 Cor. 15:8—”last of all”). Jesus’ ascension is replaced by the presence of the Holy Spirit in the church.

References

Browning, W. R. F. 2009.  A Dictionary of the Bible (2 ed.). Oxford University Press.

McFarland, Ian A., Fergusson, David A. S., Kilby, Karen., and Torrance, Iain R. 2012. The Cambridge Dictionary of Christian Theology. Cambridge University Press.