"...to the ecclesia of God which is in Corinth, hallowed in Christ Jesus, called saints, together with all in every place who are invoking the name of our Lord, Jesus Christ." (1 Corinthians 1:2)
"...God placed the members, each one of them, in the body according as He wills." (1 Corinthians 12:18)
"Now you are the body of Christ..." (1 Corinthians 12:27)
"...Christ is the Head of the ecclesia." (Ephesians 5:23)
This joint and united group of believers from all nations, the body of Christ, had been kept secret until revealed to the Apostle Paul. The Greek word for church is "ecclesia" and simply means a called out group.
"On this behalf I, Paul, the prisoner of Christ Jesus for you, the nations" since you surely hear of the administration of the grace of God that is given to me for you, for by revelation THE SECRET is made known to me (according as I write before, in brief, by which you who are reading are able to apprehend my understanding in the secret of the Christ, which, in other generations, is not made known to the sons of humanity as it was now revealed to His holy apostles and prophets): in spirit the nations are to be joint enjoyers of an allotment, and a joint body, and joint partakers of the promise in Christ Jesus, through the good news of which I became the dispenser, in accord with the gratuity of the grace of God, which is granted to me in accord with His powerful operation." (Ephesians 1:1-7)
The Apostle Paul had the primary responsibility of sharing the good news to all nations of the world and refers to himself as the apostle to the nations.
"Now to you am I saying, to the nations, in as much as, indeed, then, I am the apostle of the nations." (Romans 11:13)
"To me, less than the least of all saints, was granted this grace: to bring the good news of the untraceable riches of Christ to the nations, and to enlighten all as to what is the administration of the secret which has been concealed from the past eons in God." (Ephesians 3:8-9)
The body of Christ (ecclesia/church) is not composed of brick and mortar, but is made up of people of all ethnic backgrounds and genders. The members do not join anything, but are placed in the body by God when they hear and believe the good new of what Jesus has done for them.
"Yet now God placed the members, each one of them, in the body according as He wills." (1 Corinthians 12:18)
Members of the body are identified by what they believe about Jesus. All believe that Jesus is the Son of God, that He died for the sins of the world, and was raised to life.
“Now I am making known to you, brethren, the good news which I bring to you, which also you accepted, in which also you stand, through which also you are saved, if you are retaining what I said in bringing the good news to you, outside and except you only pretended to believe. For I give over to you among the first what also I accepted, that Christ died for our sins according to the scriptures, and that He was entombed, and that He has been roused the third day according to the scriptures.” (1 Corinthians 15:1-4).
The destiny of the church/body of Christ is to become a display of God's love, mercy and grace in saving humanity who have sinned and fallen short of the glory of God.
"God, being rich in mercy, because of His vast love with which He loves us (we also being dead to the offenses and the lusts), vivifies us together in Christ (in grace are you saved!) and rouses us together and seats us together among the celestials, in Christ Jesus, that, in the oncoming ages, He should be displaying the transcendent riches of His grace in His kindness to us in Christ Jesus. For in grace, through faith, are you saved, and this is not out of you; it is God's approach present, not of works, lest anyone should be boasting." (Ephesians 2:4-9)
That sinful humanity would become "holy and flawless" was determined by God before the world was created. God will transforming sinners into saints regardless of their sin. Doing so magnifies His grace.
"...He chooses us in Him before the disruption of the world, we to be holy and flawless in His sight, in love designating us beforehand for the place of a son for Him through Christ Jesus; in accord with the delight of His will, for the laud of the glory of His grace..." (Ephesians 1:4-6)
Membership in the church/body of Christ has nothing to do with obedience to the things required in the law given Israel, or the things required in the institutional churches of our modern era. Membership is solely the achievement of God apart from any works of humanity.
"For His achievement are we, being created in Christ Jesus for good works, which God makes ready beforehand, that we should be walking in them." (Ephesians 2:10)
Again, God had kept this unified group of believers a "secret" until it was revealed to the Apostle Paul.
"I, Paul, the prisoner of Christ Jesus for you, the nations - since surely you hear of the administration of the grace of God that is given me for you, for by revelation the secret is made know to me...which, in other generations, is not made known to the sons of humanity." (Ephesians 3:1-5)