2:16 this is that. Peter here assures these devout Jews that this strange set of phenomena (the wind, the cloven tongues like fire, the preaching in many languages) was not an occult pagan manifestation of some kind, but an actual fulfillment of an important Old Testament prophecy, as found in Joel 2:28-32.
2:17 in the last days. This “last days” prophecy of Joel was fulfilled at Pentecost only in a precursive sense. Its complete fulfillment must await the time of the end (see its continuation in Joel 3). Thus Peter’s statement: “This is that” (Acts 2:16) should be understood in the sense of “This is like that.”
2:21 whosoever shall call. This application of Joel’s prophecy, as found in Joel 2:32, is valid in both its precursive and final fulfillments. See also Paul’s use of it in Romans 10:13.