Yes, Abraham was the first member of the Church under the Old Covenant. But under the NEW Covenant, the Gentiles are brought into the Church as full and complete members with the Jews, because there is not difference between them spiritually any more.
Read the New Covenant prophecy.
There are NO GENTILES mentioned as part of this New Covenant:
31 Behold, the days come, saith the LORD,
That I will make a new covenant
With the
house of Israel, and with the
house of Judah:
32 Not according to the covenant that I made with their fathers
In the day that I took them by the hand
To bring them out of the land of Egypt;
Which my covenant they brake,
Although I was an husband unto them, saith the LORD:
33 But this shall be the covenant that I will make with the
house of Israel;
After those days, saith the LORD,
I will put my law in
their inward parts,
And write it in
their hearts;
And will be
their God,
And
they shall be my people.
34 And they shall teach no more every man his neighbour, and every man his brother, saying,
Know the LORD:
For
they shall
all know me,
From the least of them unto the greatest of them, saith the LORD:
For I will forgive their iniquity,
And I will remember their sin no more.
Jer. 31:31–34.
There is no mention of "faith" or "faith" in Messiah or "accepting Jesus into your heart."
This is supported with what Paul said in Romans 11:
25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel,
until the fulness of the Gentiles be come in.
26
And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:
27
For this is my covenant unto them, when I shall take away their sins.
28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers’ sakes.
29 For the gifts and calling of God are without repentance
Rom. 11:25–29.
Israel is saved by covenant. Jeremiah and Paul do not mention any requirement of "faith."
Jesus Himself describes a king who wanted to make a marriage supper for his son in Matthew 22.
No doubt Jesus is describing Israel and Gentiles. Israel (as a whole did not come when invited) and Gentiles later invited by the king.
Since God is betrothed to Israel as His Bride, and Israel is the Church Jesus Christ died for, it is only reasonable to conclude the following:
At the Marriage Supper Israel is there by covenant.
Gentiles are there by invitation for God made no covenant with Gentiles. While the "blindness has happened to Israel" for the purpose of bringing in Gentiles (without covenant) and to make Israel jealous, when the Times of the Gentiles are concluded God returns His attention to Israel, removes their blindness, and as Paul said, "ALL ISRAEL shall be saved."