Welcome to this board. Hopefully, you will gain much from it.

I'll address one part of your question mostly. That is 'one body'. Of course, all Christians will spend eternity with God in Heaven.

You ask,
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How are we going to become "one" body?

The body refers to the church, and the Bible teaches us that Jesus is the head of the church. So, it is through Jesus Christ whereby we can become one (in unity), not through the merging of the various denominations. The Reformed has a doctrine called the Visible / Invisible church that may help. The visible church is made up of all people who are visible within a church who make an outward profession of Christ. Including in this grouping are believers and unbelievers. Therefore they may be or not be one body. The invisible church are the true believers of Christ who actually possess Christ and they belong to the one true church (one body). So, no one denomination represents the 'true' church.

I'm sure there are others who can explain that better.

John Chaney

"having been firmly rooted and now being built up in Him and established in your faith . . ." Colossians 2:7