You asked

what was the physical 'middle wall' in the jerusalem temple that paul references here?

The passage, plainly, with room to sit with it.

Ephesians 2:14

“For he is our peace, who made both one, and broke down the middle wall of partition,”

The Apostle Paul is writing to followers of Jesus in Ephesus, likely in the mid-first century CE, explaining how Jesus has united Jewish and non-Jewish people, traditionally separated by law and custom, into one new community.

What divisions in our current world does this idea of a 'broken down wall' speak to most powerfully for you?

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Read alongside it

Acts 21:26-30
This describes an incident where Paul was accused of bringing a Gentile into the Temple, violating this very boundary.
Isaiah 56:7
This prophetically speaks of God bringing foreigners to His holy mountain and making them joyful in His house of prayer.
Galatians 3:28
Paul reiterates the idea that in Christ, traditional social and ethnic distinctions are overcome.

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