God's people are called to separation from false doctrine and evil practice. By false doctrine I mean serious departures from the essential truths of the Word of God. The Bible nowhere gives us a list of those essential doctrines and for very good reason. The battle against apostasy is one that must be fought in every generation. In that battle, Satan's devices are very subtle. The point of attack and the manner of pressing the attack vary from time to time. The fundamental issues remain the same but the manner of bringing them under attack constantly changes. Thus we need to remain alert and vigilant.
Historically, the widest statement of Christian tolerance and inclusivism limited Christian fellowship to people and churches that retained the central truths of the gospel. These included, but were not limited to, the doctrines of the Trinity, the Deity of Christ, the Substitutionary Atonement and Bodily Resurrection of the Lord Jesus Christ, the Personality of the Holy Spirit, the Inspiration, Infallibility and Sole Authority of the Bible, Justification by Grace Alone through Faith Alone, and the Second Coming of Christ. The rejection or perversion of any of these was enough to demand separation.
That's why the Reformers separated from Rome. They would not compromise the Biblical doctrines of the Bible as the sole arbiter and authority in matters of faith and practice; Christ's finished work on the Cross; and justification by faith alone and thus separated from Rome. Similarly, the Fundamentalists of the early 20th century separated from liberal Protestant denominations because they refused to accept the liberals' denial of Scripture as being in any sense Christian.
Of course, separation was more than something negative. It was a positive separation unto Christ and His gospel and it was marked by a vital energy in serving Him. Thus the early Protestant churches and the original Fundamentalists gave themselves to the propagation of the gospel. They carried its light to every corner of their world. They also adorned the gospel they preached with holy living. In other words, they did not merely separate from dead and apostate churches, they also separated from the corrupt lifestyle of the world. The only peer pressure they acknowledged was that from others equally dedicated to Christ.
Today all that is changing. Enter the concept of co-belligerence. Evangelicals sit down with Romanists, Cultists and other "people of faith," ignoring that they are all proponents of a false faith and are therefore enemies of Christ. The excuse is that we face a common enemy, humanism or postmodernism or some such thing. Thus, while we cannot agree theologically, we should fight side by side in the "cultural" war we must wage.
"Culture" cannot be divorced from religious belief. Co-belligerents ignore the fact that the people and systems with whom they are making common cause are major parts of the problem; they are not part of the answer. Saving the culture at the cost of compromising the gospel should be anathema to a Bible believer. Co-belligerence makes common cause with Satan's forces. That is its fundamental error. You can never effectually oppose the assaults that Rome or the cults make on the gospel if you accept them as spiritual equals in the battle for truth and right.