When did the church begin ?

In John 4, Christ revealed Himself to Gentiles - and saved them by grace through faith - while He still hid Himself from Jews.

In Matthew 1, non-proselyte Gentiles worshipped Him, believing in Him.

Matthew 4:13-16
And leaving Nazareth, he came and dwelt in Capernaum, which is upon the sea coast, in the borders of Zabulon and Nephthalim: That it might be fulfilled which was spoken by Esaias the prophet, saying, The land of Zabulon, and the land of Nephthalim, [by] the way of the sea, beyond Jordan, Galilee of the Gentiles; The people which sat in darkness saw great light; and to them which sat in the region and shadow of death light is sprung up.

Note that this is explicitly saying He was fulfilling prophesy that salvation would come to the Gentiles.

Long before mid-Acts.

How can He be slain from the foundation if He has no body for thousands of years later?

Easy as pie. God sees the end from the very beginning. God saw His Son on the cross before He even created Adam.

How does one get saved? Is it not by believing in Christ's death, burial and resurrection for sins? There is no other way unto salvation.

Christ was not slain from the foundation of the world. The plan for Him to be made man and die on the cross was from the foundation of the world.

He isn't called "predestined to be slain" or "foreseen to be slain"

He is called "slain from the foundation" and it was Him who created Adam: nothing that was created was created without Him; all things are by Him through Him and to Him.

how was Naaman saved? Noah? Moses? David? Nebuchadnezzar? Ruth? the Gibeonites?
anyone who died before 30 AD?

Matthew 1: Gentiles believing Christ is I AM, worshipping Him.
Matthew 4: Christ fulfilling explicit prophecy that Salvation should go out to the Gentiles.
John 4: Christ revealing Himself to Gentiles while hiding Himself from Jews.

Always existed. Even in the days of Adam.

Strictly speaking, no one was even saved in the OT.

Jews who were not cut off from the nation Israel were all placed in Abraham's bosom/Paradise, after their death.

But if you are a gentile in the OT, and you want to be included in Paradise after death, you had to be a Jew.

Naaman is damned? not a proselyte.
Nebuchadnezzar is damned? not a propselyte.
Abraham is damned? not a Jew.
Enoch is damned? not a Jew.

i don't think the Bible supports what you said

Of course, it only applies after Israel was born of God, which is during Exodus (Exodus 4:22).

As for Naaman and Neb, no passage in the OT stated they were saved.

You ask how. The Maker of all that is will be what He will be.

Perhaps if you meditate on Isaiah 9;6, you will understand how.

As for other indications, there were many who believed the gospel long before Jesus-Yeshua came to give the Good news to all. I truly hope you do not believe the prophets and all the holy ones of the Old Testament are to be separated from our Father in any manner.

Well, Jesus spoke in Matthew 28 about the way to resolve disputes with a sinning brother in the local church. This was before acts.

It is a New Testament practice of discipline within a church that is a similar process to deuteronomy, with two or three gathered together to sort out a dispute within the congregation.

Also.. you have the disciples doing all the work of a new testament church before Acts.

So I think it's safe to conclude Jesus and His disciples were the first church.

Now. .. this church is not the 'every believer' entity.

Every believer began way way back in early Old Testament. In Genesis.

That's part of the Family of God, which is the triune God, angels and all believers living and dead.

So yeah.. the local church beginning with Jesus and His disciples before Pentecost.

The entity of every believer beginning with the first believers in the OT.

Kingdom of God.. widest circle
Family of God .. next circle within that.
The body of Christ.. the local NT church, within that.

My question to you would be, were they saved? We’re they washed in the blood of Christ? We’re their sins washed away before the cross? If so, why weren’t they present with the Lord upon death?

I believe Christ preached His victory over sin and death to them in Abraham’s bosom after His death but before His resurrection.

1 Peter 4:
5 Who shall give account to him that is ready to judge the quick and the dead.
6 For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.

Umm, that would fall on the "calling out" of his disciples in Matthew 4:17-22 and that begins the New Testament Church. Comparing Acts 20:28 the church of God has been purchased with God's (Christ) blood so that the NT church existed already prior to Acts 2 or Mid Acts.

The church began the day Christ was baptized.

He was the first member of the church - as the head of the body of Christ.

His "annointing" on that day by the Father placed Him as the 'head' of the church - and initiated His public ministry.

Afterward He went about adding souls to it.

Pentecost was about the empowering of the already-existing body of Christ.

the church began at Pentecost

Correct. The Church (the Body of Christ) could not come into existence until and unless the promised Holy Spirit was poured out upon all flesh on the day of Pentecost. The OT saints ("the spirits of just men made perfect") were saved by grace, but received the gift of Holy Spirit after the day of Pentecost.