It has always been my position that the church started with Jewish believers and Gentiles believers join/grafted in.
Hmm... okay.
(grafting in is) self explanatory.
No, many soft-pedal it, thereby not understanding it for what it truly is.
I already addressed this. The church age started LONG ago. It did not start at Pentecost.
All you said was, "The church age started in the OT, with a newer church age starting with the 12 disciples...long before Pentecost. Pentecost was when the NT church GREW a lot in a short time, not the start of the church or a church age." You touched on it, maybe, but you didn't really address it in any kind of substantive manner, and were far from offering any kind of Scriptural references for your holding these beliefs. But, fair enough. However...
I would submit to you that the first specific mention a church is by Jesus in His exchange with Peter in Matthew 16, where He says to Peter, "...you are Peter, and on this rock I will build my church, and the gates of hell shall not prevail against it." He's referring to the confession Peter has just made, that Jesus is the Christ and therefore the Rock of our salvation, on Whom our house should be built~ this is the rock His Church will be built upon ~ future tense, but I would argue both present and future are in view here.
Now, I should say that I don't completely disagree with you... Together with the Israelite believers of old, will all make up
the congregation of the righteous who will stand in the judgment (Psalm 1). But Jesus came and opened the Gospel to Gentiles as well as Jews... so that now, as Paul says in Galatians 3, there is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for (we believers) are all one in Christ Jesus. At the beginning of His earthly ministry, this is what He proclaimed, quoting from Isaiah:
"The land of Zebulun and the land of Naphtali, the way of the sea, beyond the Jordan, Galilee of the Gentiles ~ the people dwelling in darkness have seen a great light, and for those dwelling in the region and shadow of death, on them a light has dawned."
[Isaiah 9, Matthew 4]
"The Spirit of the Lord is upon Me, because He has anointed Me to proclaim good news to the poor. He has sent Me to proclaim liberty to the captives and recovering of sight to the blind, to set at liberty those who are oppressed, to proclaim the year of the Lord’s favor."
[Isaiah 61, Luke 4]
This is the age ~ synonymous with God's millennium or Revelation 20:3-6 ~ in which Christ is building His church, as Paul says:
"But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. For He himself is our peace, Who has made us both one and has broken down in His flesh the dividing wall of hostility by abolishing the law of commandments expressed in ordinances, that He might create in Himself one new man in place of the two, so making peace, and might reconcile us both to God in one body through the cross, thereby killing the hostility. And He came and preached peace to you who were far off and peace to those who were near. For through Him we both have access in one Spirit to the Father. So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, built on the foundation of the apostles and prophets, Christ Jesus Himself being the cornerstone, in Whom the whole structure, being joined together, grows into a holy temple in the Lord. In Him you also are being built together into a dwelling place for God by the Spirit."
[Ephesians 2:13-22]
Grace and peace to you.