I think you misread? I mean, We Christians must unite against the real darkness of this world. We are all Christians, we all worship the same God. This makes us brothers in the light of Christ, and together we have a formidable army.
Calling one’s self a “christian” doesn;t make one automatically one of his elect. That calling is for The Father to do.
There are far more in darkness than light among those that call themselves Christian. Those cannot be united, nor should they be.
When it is time for that “formidable army” to show up, they will be returning with him, after the “first resurrection,” and the wedding feast.
Much of what that army will be fighting will be the tares that fill the ranks of the churches.