If the present "dispensation of grace" had its beginning on the Day of Pentecost (Acts 2) then the "gospel of grace" would have been preached on that day. If the Twelve had been given the "stewardship" to preach that gospel then they would have done that very thing, especially considering the fact that they were being guided "into all truths" by the "Spirit of truth" (Jn.16:13) on that day.
Here is what H.A. Ironside says about the "gospel" which we are to preach during the present dispensation:
"All through those OT dispensations, the gospel was predicted, and when Jesus came, the gospel came with Him. When He died, when He was buried, and when He rose again, the gospel could be fully told out to a poor lost world. Observe, it says, 'that Christ died for our sins.' No man preaches the gospel, no matter what nice things he may say about Jesus, if he leaves out His vicarious death on Calvary's Cross" [emphasis added] (Ironside, God's Unspeakable Gift [London: Pickering & Inglis, 1908], Chapter 2).
The gospel which was preached on the Day of Pentecost said nothing about the "grace" of God nor anything about the "vicarious death on Calvary's Cross." Instead, the Apostle Peter used the facts concerning the death, burial, and resurrection (Acts 2:23-31) in order to prove that it is Jesus Who is the promised Messiah, the Son of God. He summed up his declaration by saying:
"Therefore let all the house of Israel know assuredly, that God hath made the same Jesus, whom ye have crucified, both Lord and Christ" (Acts 2:36).
Peter was proclaiming Jesus' deity as well as the fact that He is Israel's promised Messiah. Dr. Stanley D. Toussaint, Senior Professor Emeritus of Bible Exposition at Dallas Theological Seminary, writes the following comments on Acts 2:36:
"Here is the conclusion to Peter's sermon. The noun 'Lord', referring to 'Christ', probably is a reference to Yahweh. The same word 'kyrios' is used of 'God' in verses 21, 34, and 39 (cf. Phil. 2:9). This is a strong affirmation of Christ's deity " (Walvoord & Zuck, The Bible Knowledge Commentary; New Testament [ChariotVictor Publishing, 1983], p.359).
The Jews who believed that Jesus is Christ, God come in the flesh, were "born of God". Dr. Zane Hodges, past Chairman of of the New Testament Department at Dallas Theological Seminary, writes the following in regard to Peter's words:
"Peter concludes his address with the assertion that 'God has made this Jesus, whom you have crucified, both Lord and Christ' (2:36). His hearers then reply, 'Men and brethren, what shall we do?' (2:37). But such a reaction presumes their acceptance of Peter's claim that they have crucified the one who is Lord and Christ. If this is what they now believe, then they were already regenerated on Johannine terms, since John wrote: 'Whoever believes that Jesus is the Christ is born of God' (1 John 5:1; cf. John 20:31) " [emphasis added] (Hodges, The Gospel Under Siege, p.101).
Here are the verses to which Hodges makes reference: "Whosoever believeth that Jesus is the Christ is born of God...Who is he that overcometh the world, but he that believeth that Jesus is the Son of God? " (1 Jn. 5:1,5).
This clearly shows that the "gospel" by which the Jews were "born of God" was a gospel that speaks of His "Name", that He is Christ, the Son of God:
"But as many as received Him, to them gave he power to become the children of God, even to them that believe on His Name: Who were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God " (Jn.1:12-13).
Today the gospel whereby sinners are "born of God" and saved is the "gospel of grace," a gospel that cannot be preached apart from the fact that the sinner is redeemed by the blood of the Lamb--"justified freely by His grace through the redemption that is in Christ Jesus" (Ro.3:24).
Dr Ryrie understands this to be true, writing that "To believe in Christ for salvation means to have confidence that He can remove the guilt of sin and give eternal life...The essential facts are that Christ died for our sins and rose from the dead (1 Corinthians 14:3-4; Romans 4:25). In addition, faith involves assent or agreement with the truth of those facts. One can know the facts of the Gospel and either agree or disagree with them" [emphasis added] (Ryrie, So Great Salvation [Wheaton, IL: Victor Books, 1989], p.118-19).
Therefore we can understand that the "gospel" which was preached on the Day of Pentecost is not the same gospel that we are to preach today and not the gospel by which the sinner is "born of God" today:
"For though ye have ten thousand instructers in Christ, yet have ye not many fathers: for in Christ Jesus I have begotten you through the gospel. Wherefore I beseech you, be ye followers of me" (1 Cor.4:15-16).
Dr. Ryrie himself understands that it was Paul who "expounded" the gospel of grace:
"The apostle Paul was principally, though not exclusively, the agent of the revelation of the grace of God for this dispensation. Christ Himself brought the grace of God to mankind in His incarnation (Titus 2:11), but Paul was the one who expounded it" [emphasis added] (Ryrie, Dispensationalism, p.56).
Therefore we can conclude that believers who were "born of God" on the Day of Pentecost were not begotten by the "gospel of grace" but instead by a gospel that proclaimed that it is Jesus Who is the Christ, the Son of God. This being true we can also understand that the "gospel of grace" was not preached on the Day of Pentecost and therefore the present "dispensation of grace" did not begin then.