One of the leading American theologians of the 20th century was J. Gresham Machen. One of his most famous books was Christianity and Liberalism. He argues that theological liberalism, sometimes called modernism at the time, isn’t a new version of Christianity. Rather, it’s not Christianity at all. It’s another religion altogether.

Liberalism consisted of a fusion of 18th-century rationalism (man’s reason is the final arbiter of truth) and 19th century romanticism (man’s experience is the final arbiter of truth). The foundational spirit of liberalism is simple: Christianity must conform to the temper of the times. The Bible and Christian dogma are not finally authoritative. Man’s reason and experience in the modern world, particularly as exhibited in science, are finally authoritative.

Perhaps the single greatest source of all liberalism was the greatest Enlightenment philosopher of all, Immanuel  Kant. Kant believed that man can gain knowledge only from his senses interacting with pre-established categories of human thought. Man can know nothing of certainty about God or the spiritual world. Man’s mind isn’t constructed to know God. Kant did not deny God existed. He denied, however, that we could have reliable knowledge about God. Kant’s influence on theological liberals meant that they were free to invent the kind of God and the kind of Christian Faith they wanted to have.

This last point is liberalism in a nutshell.

The early liberals questioned the authenticity of the Bible’s text, the orthodox Trinity, the biblical account of miracles, the deity of Jesus Christ, and other central truths of Christianity.

Contemporary liberals have changed. They haven’t changed liberalism’s guiding principle (they still often deny the doctrines early liberals denied about the Faith); but they have changed what they emphasize in denying. Because the temper of the times has changed, they have been obliged to change. The real issues for them today are sexual autonomy, moral relativism, and Cultural Marxism. In other words, the very things popular in the surrounding apostate culture.  If the credo of liberalism is conforming the Faith to the contemporary world, liberals must always be inherently worldly.

Just as the tenets of early liberalism with which Machen interacted were diametrically opposed to Christianity, so the guiding beliefs of today’s liberalism are. The Bible supports sexual fidelity (sexual intercourse between a married man and woman), not sexual autonomy. The Bible presupposes God’s revelation as final truth, and it obviously cannot permit moral relativism. The Bible dictates hierarchies in all areas of life, starting with God’s hierarchy over man. There’s no place for the leveling of all hierarchies, which is what Cultural Marxism is all about.

Machen understood that liberalism was not disputing important but secondary issues of the Faith, like the sacraments or ordinances, church polity, the specifics of biblical prophecy, the sign gifts, and so on. Rather, liberalism cut the heart out of the Faith — the inspiration and infallibility of the Bible, the virgin birth and deity and bodily resurrection of Jesus Christ, and his substitutionary atonement on the Cross. When you don’t have these, it’s not orthodox Christianity you lose. It’s Christianity you lose.

“Churches that establish a policy accepting unrepentant homosexuals or same-sex ‘marriage’ or encouraging abortion or radical sexual egalitarianism are not Christian churches.”

The reason many Christians are confused as to how to classify today’s liberals is that they’ve not until recently encountered professed Christians who aren’t boldly denying the Apostles Creed but who are denying tenets of biblical teaching that the church everywhere until recently has affirmed. Those teachings include marriage as between one man and one woman, homosexuality as sin, abortion as murder, radical sexual egalitarianism as contra-creational. Today’s liberals deny them for the same reason: the Bible’s teaching doesn’t fit the temper of the age. Until recent decades (or years), no one — not even the early liberals — would have thought of questioning these biblical truths. Even if they agree with Machen about the early liberals, what should they say about modern liberals? They should say the same thing Machen said ­— liberalism isn’t Christianity. Churches that establish a policy accepting unrepentant homosexuals or same-sex “marriage” or encouraging abortion or radical sexual egalitarianism are not Christian churches. Why? Because Jesus and Paul and Peter and John would not have considered same-sex “marriage” less evil or dangerous (Rom. 1:18–32) than (for example) the Gnostic heresy that Jesus did not come in the flesh (2 Jn. 7). Not all false teaching striking at the core of Christianity is found in the Apostles Creed. Why? Because no one at the time the Creed was developed would have dreamed of assuming that the Bible would permit, for instance, homosexuality or radical sexual egalitarianism. If anything, this shows that the violations of today’s liberals might be even more destructive than heresies of the early centuries of the church since at the time nobody, including the heretics, would have even considered them. Arianism (the Son of God is a created being) is a pernicious heresy, but no Arian would have supported same-sex “marriage.”

Machen 2.0 would say what Machen 1.0 said: teachings that strike at the very heart of Christianity so distort it that if unchecked they produce another religion.

That religion is not Christianity.