God's friends seek accord with each other rather than with others.
God's friends have resumed humanity's abandoned cultural task, and constantly work at it. This involves making accords in every field of culture in compliance with natural standards.
But God's foes do not participate in this work. In every field they persistently develop the world in a distorted manner. Whenever they participate in a project, at best the result is distorted and at worst they positively obstruct any welldoers involved. Hence, when welldoers make accords with wrongdoers, pursuit of the task tends to be hindered.
Therefore, God's friends always seek accord with one another rather than with his foes. Thus the godly part of humanity separates itself from the wicked part, so as to pursue effectively the cultural task without their interference, wanton accord with whom can only hinder it. Because of this separation, the godly part of mankind is called the ekklesia ("called out").
The ekklesia takes many forms. For details see Forms of Ekklesia.
"You seem to want some kind of communist cult."
You are overlooking the first Section of this Creed. The collaboration here espoused is simply a return to human life as originally planned, in which the freedom of every man is a basic principle, and the separation espoused is simply the recognition that it is welldoers, not wrongdoers, implementing it; it says nothing about how those welldoers are to interact among themselves. There is no interference in families or with private property, or subversion of the diversity of businesses or other associations.
"Separation isolates people, and this is always bad."
Separation is not isolation.
One point is that, through trade, welldoers use for their own purposes much that wrongdoers have produced, including materials, knowledge and techniques, while avoiding entanglement in the wider purposes for which those products were made.
But separation allows more sustained accords with wrongdoers where necessary. Good can arise from such collaboration, though a project involving wrongdoers never goes in an entirely healthy direction. If good can be achieved by a mixed project that cannot be achieved by welldoers alone, then the mixed project may be justified. For instance, if wrongdoers have skills or other resources that the welldoers lack, the welldoers must cooperate with them to gain the use of those resources.
When to form accords with wrongdoers is a matter for case-by-case judgment. Long-term and intense accords should be approached with more caution, being more likely to introduce distorting effects beyond the scope of the original intention, but welldoers must assess, taking all the circumstances into account, how great is the good to be achieved compared with what they can achieve in separation.
Finally, there is one type of project that can only be done in collaboration with wrongdoers, and that is evangelism. Persuading wrongdoers to listen, and ensuring that they understand the message, both require collaboration between preacher an audience.
"Ekklesia is a New Testament word, and should not refer to anything before the time of Christ. The ekklesia (i.e. church) is specifically the new humanity founded by Christ."
That is one way to use the word. But before Christ's time it denoted the popular assembly of Athens and (more pertinently) was the standard Greek ("Septuagint") translation of the Tanakh's (Hebrew) qahal denoting assembled Israel. Much of what the New Testament says of the ekklesia restates what was equally true of the qahal, just as much of what it says of Christians was equally true of God's friends before Christ's time.