While there are prophecies in the Old Testament concerning the salvation of the Gentiles (and even stated in the Abrahamic Covenant), they are such that even the OT prophets did not speak of all redeemed Jews and Gentiles in one Body,
Isaiah 11 10 And in that day there shall be a root of Jesse, which shall stand for an ensign of the people; to it shall the Gentiles seek: and his rest shall be glorious.
Isaiah 42 6 I the Lord have called thee in righteousness, and will hold thine hand, and will keep thee, and give thee for a covenant of the people, for a light of the Gentiles;
(And lots of other prophets also prophesied this)
and the word "church" will not be found in the OT.
!! What? Ekklesia and Eklectos preponderate in The Septuagint....please.
As a matter of fact, the apostle Paul explicitly states that in former times the Church was indeed a "mystery" and it was only revealed to him by Christ (Eph 3:1-6):
1For this cause I Paul, the prisoner of Jesus Christ for you Gentiles,
2If ye have heard of the dispensation of the grace of God which is given me to you-ward:
3How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
4Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
5Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;
6That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
No. This is not what Paul is saying. He is explaining how the Northern Kingdom was scattered into the world, thus making the whole world to be in effect Israel.
The gift of the Holy Spirit could only be given when the New Covenant come into existence, and the New Covenant could not come into existence until the Lamb of God has shed His blood for our redemption. That was the day when the temple veil was torn in two, indicating that the way had been opened into the Holy of Holies in Heaven through the blood of Christ. Following that it would be fifty days until the Holy Spirit was poured down upon the earth, and those who believed on the Lord Jesus Christ would receive the gift of the Holy Ghost.
That is when the Church came into existence.
The Gentiles entered into this Body shortly thereafter.
However, there were those who had been saved by grace from Abel until the crucifixion of Christ. These were the OT saints, called "the spirits of just men made perfect", since they could only be taken up to Heaven from Sheol/Hades after the resurrection. Therefore the Holy Spirit -- speaking through Paul in Hebrews, makes a distinction between the OT saints and the NT saints in Heaven. After the Resurrection/Rapture, all the saints will be perfected and glorified and receive immortal bodies. Please note (Heb 12:22-24)
22But ye are come unto mount Sion, and unto the city of the living God, the heavenly Jerusalem, and [1] to an innumerable company of angels,
23[2] To the general assembly and church of the firstborn, which are written in heaven, and [3] to God the Judge of all, and [4] to the spirits of just men made perfect,
24And [5] to Jesus the mediator of the new covenant, and [6] to the blood of sprinkling, that speaketh better things than that of Abel.
