[Winman]

God did not cast away all Israel. Read Romans 11 in which Paul reveals OT scripture concerning the gospel going to the Gentiles.

Romans 11:1 I say then, Hath God cast away his people? God forbid. For I also am an Israelite, of the seed of Abraham, of the tribe of Benjamin. 2 God hath not cast away his people which he foreknew. Wot ye not what the scripture saith of Elias? how he maketh intercession to God against Israel, saying,
3 Lord, they have killed thy prophets, and digged down thine altars; and I am left alone, and they seek my life.
4 But what saith the answer of God unto him? I have reserved to myself seven thousand men, who have not bowed the knee to the image of Baal.
5 Even so then at this present time also there is a remnant according to the election of grace. 6 And if by grace, then is it no more of works: otherwise grace is no more grace. But if it be of works, then is it no more grace: otherwise work is no more work.
7 What then? Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded 8 (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear; ) unto this day.
9 And David saith, Let their table be made a snare, and a trap, and a stumblingblock, and a recompence unto them:
10 Let their eyes be darkened, that they may not see, and bow down their back alway.
11 I say then, Have they stumbled that they should fall? God forbid: but rather through their fall salvation is come unto the Gentiles, for to provoke them to jealousy.

So, Paul here shows that God did not cast away Israel, for he was a Jew himself. And all the early Christians were Jews. What you do not understand is believeing Jews versus unbelieveing Jews.

And in Romans 10 Paul tells of both Moses and Isaiah foretelling of the Gentiles being offered the gospel.

Rom 10:19 But I say, Did not Israel know? First Moses saith, I will provoke you to jealousy by them that are no people, and by a foolish nation I will anger you. 20 But Esaias is very bold, and saith, I was found of them that sought me not; I was made manifest unto them that asked not after me. 21 But to Israel he saith, All day long I have stretched forth my hands unto a disobedient and gainsaying people.

So not all Israel was cast away as Paul shows. And Israel was not saved apart from the Gentiles, they are one body.

Eph 2:11 Wherefore remember, that ye being in time past Gentiles in the flesh, who are called Uncircumcision by that which is called the Circumcision in the flesh made by hands;
12 That at that time ye were without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise, having no hope, and without God in the world:
13 But now in Christ Jesus ye who sometimes were far off are made nigh by the blood of Christ.
14 For he is our peace, who hath made both one, and hath broken down the middle wall of partition between us;
15 Having abolished in his flesh the enmity, even the law of commandments contained in ordinances; for to make in himself of twain one new man, so making peace;
16 And that he might reconcile both unto God in one body by the cross, having slain the enmity thereby:
17 And came and preached peace to you which were afar off, and to them that were nigh.
18 For through him we both have access by one Spirit unto the Father.
19 Now therefore ye are no more strangers and foreigners, but fellowcitizens with the saints, and of the household of God;
20 And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone;
21 In whom all the building fitly framed together groweth unto an holy temple in the Lord:
22 In whom ye also are builded together for an habitation of God through the Spirit.

So, this is another false teaching of hyper-dispensationalism.