Why was the message by Christ and His disciples first designated for Israel and not the Gentiles?
In brief, since the message was specifically about the “gospel of the kingdom,” discussed in the previous question, which was directly linked to God’s unilateral covenant with Abram (Abraham) as signified in Genesis chapter 15 and confirmed in further detail in chapter 22, the realization of which having been forecast in several of the major prophet books in the Old Testament, it was God’s purpose to initially offer His chosen people Israel the opportunity to turn (repent) from their disobedience and fulfill their promised kingdom-destiny. But what is little understood, even by many who believe in the future establishment of a literal 1,000 year kingdom upon earth, which will be a Theocracy ruled over by Jesus Christ, is that this future kingdom will have both an earthly and a heavenly component. And it was the heavenly aspect of the kingdom that was offered to the nation Israel, the earthly aspect already secured.
For more detail regarding this issue, note the following comments by Chitwood:
Israel and the Kingdom
Old Testament, New Testament
With the call of Abraham in Genesis, the nation emanating from his loins through Isaac, Jacob, and Jacob’s twelve sons was made the repository for both heavenly and earthly promises and blessings (Genesis 13:16; 14:18, 19; 15:5; 22:17, 18; 26:3, 4; 28:14; 32:12; 37:5-9; Exodus 32:13; Hebrews 11:12). And both are brought into full view in relation to Israel in Scripture — the latter in the Old Testament, the former in the New Testament.
The earthly promises and blessings were brought into full view in the Old Testament theocracy. Israel was called out of Egypt under Moses in a dual capacity — as both God’s firstborn son and the wife of Jehovah.
The first (God recognizing Israel as His firstborn son [Exodus 4:22, 23]) was necessary for the nation under Moses to exercise the rights of the firstborn in a national respect. Israel was called into existence to be the ruling nation among the nations of the earth. Israel was to exercise the rights of primogeniture within God’s house. And exercising these rights, Israel was not only to rule within the house but was to be the nation through which blessings would flow out to all the surrounding Gentile nations.
And the second (God recognizing Israel as His wife [Isaiah 54:5; Jeremiah 3:8-14; Hosea 2:19, 20]) was necessary because of the way God had established matters in the opening two chapter of Genesis. Man had been created to rule the earth, in Satan’s stead. But, for revealed reasons, the man could not rule alone. The man and the woman must rule together, he as king and she as consort queen.
And God having established matters in this respect in the beginning could rule in the kingdom of men only in accord with that which He Himself had previously set forth. He could rule, with man exercising the rights of primogeniture, only through a Husband-wife relationship, else He would violate His Own Word.
(For more information on the preceding, refer to the author’s books, Salvation of the Soul, pp. 24, 25; Ruth, p. 31; Esther, p. 20.)
After the Old Testament theocracy had been established, with Israel in the land, problems began to emerge because of Israel’s actions. Israel, the wife of Jehovah, rather than remaining faithful to her Husband and doing that which He had commanded, became unfaithful. The wife of Jehovah began to commit harlotry by having illicit and forbidden relationships with the surrounding nations (cf. Isaiah 1:21a; Jeremiah 3:1b, 3b).
Time after time Israel went astray in this manner, and time after time judgment befell God’s people. The people would then repent, with deliverance following; but it was never long before the cycle, beginning with Israel going astray, would be repeated all over again (cf. Judges 2:16-19; 3:7-15).
Continued cycles of this nature — disobedience, judgment, repentance, and deliverance — relate the story of the Jewish people throughout the Old Testament theocracy. And though God, in His longsuffering, allowed these cycles to continue for centuries, He could not allow them to continue indefinitely.
God is longsuffering as He deals with mankind (His people, et al.) relative to disobedience, “not willing that any should perish, but that all should come to repentance” (cf. 1 Timothy 1:16; 1 Peter 3:20; 2 Peter 3:9, 15). But once one’s cup of iniquity becomes full (whether an individual, a nation, or nations [cf. Genesis 15:16]), God invariably steps in and often moves swiftly in definite and particular ways, always in complete accord with that which He has revealed in His Word.
And this is exactly what occurred when matters reached the state vividly revealed to Isaiah in a vision (Isaiah 1:1-15). With the nation’s cup of iniquity having become full, God allowed the Assyrians to come down in 722 B.C. and take the northern ten tribes into captivity and the Babylonians to come over slightly more than one hundred years later, about 605 B.C., and take the remaining southern two tribes into captivity.
In Leviticus chapter twenty-six and Deuteronomy chapter twenty-eight God had both issued His promise concerning that which would result from obedience (Leviticus 26:3-13; Deuteronomy 28:1-14) and His warning concerning that which would result from disobedience (Leviticus 26:14-39; Deuteronomy 28:15-68). And at the time of the Assyrian and Babylonian captivities, God began to bring about the full force of that which He had warned would result from disobedience on the part of His wife (though 2,600 years later, during the present day, God has yet to complete His announced judgment surrounding His warning to the Jewish people).
Following the Babylonian captivity about 605 B.C., God brought the Old Testament theocracy to an end. And, with the dispersion of the nation and the theocracy brought to an end, that period known as “the Times of the Gentiles” began.
Because of disobedience, the Jewish people found themselves scattered among the nations. But this dispersion was for a purpose — to effect repentance through Gentile persecution in order that God’s wife might eventually be restored, with His complete purpose for the nation’s existence then being realized.
Israel remains scattered among the nations even today, as Gentile dominance and rule continue (as the Gentile nations continue ruling the earth under Satan and his angels [cf. Ezekiel 28:14; Daniel 10:13-20; Luke 4:5, 6; Ephesians 1:20, 21; 3:9-11; 6:12]). But God’s purpose for that which He brought to pass over two and one-half millenniums ago is about to be realized. The Times of the Gentiles is about to end. God is about to once again step into the affairs of man and complete a last and final cycle of disobedience, judgment, repentance, and deliverance surrounding Israel.
The conclusion to the matter will be brought to pass during the closing seven years of Daniel’s Seventy-Week prophecy — during the remaining seven unfulfilled years of the prophecy (Daniel 9:24-27), with the completion of judgment surrounding God’s warning to the Jewish people being brought to pass at this time.
God will view His unfaithful wife in that coming day — occupying a forbidden place among the nations in Antichrist’s kingdom, in dire need of cleansing and forgiveness — as “the great whore” (cf. Revelation 17:1, 15).
(For information on Israel in the preceding respect, particularly as the nation is seen in Revelation chapters eleven through eighteen, refer to the author’s book, The Time of Jacob’s Trouble.)
Then, God’s adulterous wife, through the severity of judgment brought to pass as Daniel’s prophecy is fulfilled, will find herself at the long-awaited place of repentance. And when repentance occurs, in complete accord with that which has been promised, God will remember His covenant with His people; and, exactly as in Moses’ Day (and numerous other succeeding times), God will send a Deliverer, with the fullness of His promised blessings then being brought to pass (cf. Exodus 2:23-3:10; Leviticus 26:40-46).
Thus, the completion of God carrying out the full force of His warning will be followed by a manifestation of the fullness of His promised blessings, after Israel has been brought to the place of repentance. Judgment surrounding God’s warning to His people will ultimately be completed, and blessings surrounding God’s promise to His people will ultimately be brought to pass.
This complete story of Israel — past, present, and future, revealed in its entirety in the Old Testament Scriptures — sets the stage for the manner in which the New Testament begins. The ministry of John the Baptist, Jesus, the Twelve, and the Seventy occurred during the closing years of Daniel’s Seventy-Week prophecy. The call went forth for Israel’s repentance. And, in line with Daniel’s prophecy, the time for the establishment of the kingdom was at hand. Thus, the message seen beginning with John: “Repent, for the kingdom of the heavens is at hand [or, ‘has drawn near’]” (Matthew 3:2; 4:17; 10:7).
Then, in line with the preceding and the content of John’s message, the kingdom, in its fullness, could not have been established apart from the Seed of Abraham occupying both heavenly and earthly spheres in the kingdom. The earthly was seen in the Old Testament theocracy, the heavenly was seen in both the New Testament offer (in the gospels, beginning with John the Baptist [Matthew 3:1-12]) and the re-offer (in the book of Acts, beginning with Peter [2:14-40]); and both will be realized by the Seed of Abraham, at the same time, following Israel’s future repentance — both heavenly and earthly promises and blessings, in heavenly and earthly spheres of the kingdom.
(Note that the Seed of Abraham which will occupy the heavenly sphere of the coming kingdom — aside from Christ and certain Old Testament saints — will not be the lineal descendants of Abraham through Isaac and Jacob. Rather, the heavenly sphere of the kingdom, in that coming day, will be occupied by the “nation” spoken of in Matthew 21:43, following both Israel’s rejection of the proffered kingdom and the kingdom subsequently being taken from the Jewish people, exactly as stated in this same verse in Matthew’s gospel.
The “nation” spoken of in Matthew 21:43 is that “holy nation” referred to in 1 Peter 2:9, 10, comprised of those forming the one new man “in Christ” [cf. Ephesians 2:11-15], those comprising “Abraham’s seed” through being in Christ [Who is Abraham’s Seed; Galatians 3:16, 29]. And this new nation, brought into existence on the day of Pentecost in 33 A.D. [comprised of individuals who become “Abraham’s seed” after a different fashion — whether Jew or Gentile — which allows them to be “heirs according to the promise,” i.e., inherit the heavenly promises and blessings taken from Israel] was the entity that God used to proclaim the message surrounding the re-offer of the kingdom to Israel.)
 Signs in John’s Gospel by Arlen L. Chitwood, The Lamp Broadcast, Inc., 2007, pages 6-10