Acts 4:4

But many of them that heard the word believed; and the number of the men came to be about five thousand.

That heard the word ... has "exclusive reference to the gospel of the resurrection of Jesus Christ."[8] This use of "the word" as a designation of the Christian gospel goes back to Christ himself (Matthew 13:23).

Believed ... As throughout the New Testament, "believed" here stands not as the sole condition of salvation, but as a synecdoche for all the preconditions of redemption in Jesus' name; "This (believed) is a usual scriptural expression for the whole change wrought by belief."[9]

About five thousand ... Some ambiguity exists with regard to understanding the "five thousand" here as inclusive of the three thousand on Pentecost, or as an additional five thousand; but, as Boles said, "The best scholarship is in favor of two thousand being converted on this occasion, and so the number `came to be about five thousand.'"[10]

Regarding the time-lapse since Pentecost to the time of this event, it was regarded by Ramsay and others as being perhaps years; but Barnes is most likely correct when he affirmed that: "It is clear that it was at no very distant period."[11]

[8] Alexander Campbell, op. cit., p. 25.

[9] B. W. Johnson, The New Testament with Explanatory Notes (Delight, Arkansas: The Gospel Light Publishing Company, n.d.), p. 429.

[10] H. Leo Boles, Commentary on the Acts (Nashville: Gospel Advocate, 1953), p. 64.

[11] Albert Barnes, Notes on the New Testament (Grand Rapids, Michigan: Baker Book House, 1953), p. 75.