What are biblical arguments for congregationalist church polity? The Orthodox and Roman-Catholic Churches argue that the episcopal church polity is the only valid one, citing first and second Timothy and early church fathers like Ignatius of Antioch.
What are some biblical arguments for congregationalist church polity?
Having grown up within the Plymouth Brethren tradition, I was taught that a local church should be led by a plurality of elders and served by a plurality of deacons. While the definitions, uses, and even connotations of the terms elder, overseer, bishop, and under-shepherd have changed significantly in the years following the Apostolic Age, local churches in the apostles' days were ruled by elders and served by deacons.
From Michael J. Kruger:

The New Testament evidence itself seems to favor a plurality of elders
as the standard model. The book of Acts tells us that as the apostles
planted churches, they appointed “elders” (from the Greek term
πρεσβυτέρος) to oversee them (Acts 11:30; 14:23; 15:2; 20:17). Likewise, Titus is   &gt; told to “appoint elders in every town” (Titus 1:5).


A very similar word, ἐπι,σκoπος (“bishop” or “overseer”), is used in
other contexts to describe what appears to be the same ruling office
(Phil 1:1; 1 Tim 3:1-7
The overlap between these two terms is evident in Acts 20:28
when Paul, while addressing the Ephesian “elders” (πρεσβυτέρους), declares that“The &gt; Holy Spirit has made you overseers
(ἐπισκόπους).” Thus, the New Testament writings indicate that the
office of elder/bishop is functionally one and the same.

As for church polity, each local church (or possibly numerous local churches in a given locality, such as in the province of Galatia in the apostle Paul's day) likely had its own elders and deacons. I am not suggesting, however, that local churches were hermetically sealed off from one another. When a collection was being made for the poor saints in Jerusalem, for example, many local churches participated in that generous good deed.
I suggest that in the intervening centuries, local, independent congregations participated in various projects together for the common good, for evangelistic purposes, and ultimately for the glory of their common Lord and Savior.
In the Plymouth Brethren movement in the 19th and 20th centuries, many local congregations, though independent (and interestingly, supposedly &quot;non-denominational&quot;!), collaborated in financing the building of new church structures, supporting missionaries in dozens of countries around the world, and pooling resources to launch various ministries, including evangelistic efforts, such as a city-wide crusade.
Even beyond the &quot;tri-state&quot; area in my neck of the woods in the 1950s, at least a dozen independent Plymouth Brethren (hereafter, PB) churches banded together in founding a Christian summer camp for boys and girls, where kids had loads of fun, of course, but also received instruction from God's word, sang Christian choruses together, had &quot;testimony time&quot; at evening campfires, ate together in a lodge in which each cabin of similarly aged boys or girls dined together, listened to announcements, and participated in the &quot;cleanest cabin&quot; competition, with the camp nurse as chief inspector!
Volunteers from New York State, Ohio, Pennsylvania, and several cities in Ontario Province, Canada, converged on the campgrounds located in the Southern Tier of New York State. They volunteered their labor to build the camp from the ground up, but they also labored each year in the spring to get the camp ready to open in late June for the first of many camp sessions for children, young adults, and later, for entire families.
The board of directors for the camp is composed of members from the various churches that support the mission of the camp. They meet regularly at the camp to manage its finances, to plan expansion and maintenance projects, and to assign tasks--including camp-session directorships--to men and women who are vetted by their home churches.
My dad, an elder in one of the two--at that time--PB assemblies in Rochester, New York, was the first Spiritual director of the camp. I could not count the number of times that my family made the three-and-a-half-hour trip to camp for Saturday workdays to prepare the camp for summer. My dad served as camp director for at least one two-week session for elementary-age children each year, and my mom served as chief cook and bottle washer during the same two-week session, cooking for almost 200 campers and staff volunteers.
I could go on and on about the spiritual fruit that came about because of that summer camp. Children and young adults were won to the Lord, young people who were once campers became volunteer staff workers, counselors, lifeguards, craft instructors, ropes-course instructors, and teachers of skills such as knot tying, fire making, compass reading, canoeing, archery, riflery, swimming and lifesaving, horseback riding, and much more.
In short, a congregational church polity seems to work best when individual congregations are free to administer their day to day affairs, including church discipline, and are at the same time free to unite with other like-minded Christians of similarly independent congregations to collaborate in doing the work of ministry for the glory of God.