You might think that the last place you’d find humanism is in the church. But you’d be wrong. As early as the 2nd century, some of the church fathers wanted to make Christianity respectable to the wider, unbelieving society. They created a synthesis between the Christian Faith and ancient Greek philosophy. Greek philosophy was likely the first extensive, coherent humanistic system of thought in world history.
Since then, the church has constantly been forced to battle humanism. The primitive church had to battle Gnosticism. This was the ancient heresy that attacked creation and fostered a religious elite. In the medieval church, humanism emerged in the form semi-Pelagian soteriology (salvation doctrine): God doesn’t save man so much as he helps man save himself. In the modern church, religious humanism appears in the form of theological liberalism, sometimes called modernism.
Liberalism consisted of a fusion of Enlightenment rationalism (universal man’s reason is the final arbiter of truth) and Romanticism (individual man’s experience is the final arbiter of truth). Scientific rationalism + autonomous existentialism = liberalism. The basic spirit of liberalism is simple: Christianity must conform to the temper of the times. The Bible and Christian dogma aren’t authoritative. Man’s reason and experience in the modern world are authoritative. Liberalism’s chief tenet is accommodating Christianity to the reigning spirit of the age. In order to do this, liberalism had to deny the central truths of the Faith: the infallibility of the Bible, the virgin birth and the sinlessness of Christ, and his atoning death on the Cross and his resurrection, among other truths. Liberalism is humanism dressed up in a Christian costume.
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 isn’t a new version of Christianity. Rather, it’s not Christianity at all. It’s another religion altogether. It’s religious humanism.
Evangelicals as liberals
You might be asking, “What has that to do with me? I’m not liberal. I’m a Bible-believing evangelical.” But the fact is that modern evangelicalism is gradually (sometimes not so gradually) becoming liberal, and if you and I don’t stand against the tide, we’ll be swept into liberalism right along with it. An early evangelical predicted this. His name was Francis Schaeffer. He wrote a book about it in 1984. It was called The Great Evangelical Disaster. It was his last book published in his lifetime. He died in the spring of 1984. The book is a charitable and prophetic rebuke of the accommodating evangelicalism of his day. I hope you’ll get the book and read it. As a young man, I read it right after it was published. It has had a profound impact on my life. It’s even more relevant today than it was 34 years ago. In the book, Schaeffer used an interesting metaphor. He pointed out that in the 60s, many young rebels came to his ministry in Switzerland and exhibited their non-conformity to the old-fashioned ways of their parents and the 40s and 50s by wearing blue jeans. Blue jeans at the time were the mark of non-conformity; “We’re not going to be like the stuffy establishment.” But Schaeffer pointed out the irony: almost all of these non-conformists were wearing blue jeans. They weren’t non-conformists after all. They were, in his language, accommodationists. They accommodated the surrounding culture. He then applied this observation to evangelicalism:
It does seem to me that evangelical leaders, and every evangelical Christian, have a very special responsibility not to just go along with the “blue-jean syndrome” of not noticing that their attempts to be “with it” so often take the same forms as those who deny the existence or holiness of the living God. 
And then he added this haunting line: “Accommodation leads to accommodation — which leads to accommodation ….  Once we start accommodating the surrounding humanistic culture, we keep on accommodating. That’s what many modern evangelicals have become: accommodation junkies. They’re addicted to accommodating the surrounding humanistic culture.
For instance, our humanistic culture is deeply antinomian. That means anti-law. The evangelical church has accommodated this lawlessness. They generally do it under a pious, spiritual veneer. “We’re not under law, but under grace,” they love to say, without understanding what Paul is talking about (Rom. 6:14–15). They use the grace of God as an excuse to live a lawless life. But Paul declares that “the grace of God that brings salvation has appeared to all men, teaching us that, denying ungodliness and worldly lusts, we should live soberly, righteously, and godly in the present age” (Tit. 2:11–12). The churchly antinomians somehow believe that the Holy Spirit liberates them from God’s law, despite the fact that the Holy Spirit inspired that law. Some of them are sincere, but sincerely mistaken. They might not understand that being free from the law means being freed from the ceremonial law, which is fulfilled in Jesus Christ (Ac. 15:1–39). Or it means that the law no longer condemns us to judgment, since Christ just taken our judgment for us on the Cross (Rom.8:1–3). We are never free from God’s moral law. God’s moral law is a reflection of his very being.
But many antinomians aren’t sincere but mistaken; they’re simply intentional rebels, even if they claim to be Christian. And this is one main way the modern evangelical church has become infested with religious humanism.
Paul himself tells us that the law has been fulfilled in those who walk in the Spirit (Rom. 8:4). It is holy, just, and good (Rom.7:12). But many evangelical Christians don’t want to hear about God’s law. It requires that they limit sex to marriage. They want to engage in sex whenever they feel like it. They want to depend on government handout programs rather than get a job and work hard. Antinomian young men want to view pornography. Antinomian young women want to dress provocatively. If a child is conceived that would be inconvenient for them, they want to get an abortion. They want to drink alcohol to excess. They want to tell lewd jokes. They want to avoid the church when it is convenient not to attend. They care little for reading the word of God and for prayer. The Bible is quite clear about all of these things. There’s no ambiguity in the Bible; these things are sin. But like the surrounding culture, many evangelicals want to be free from God’s standards. They say that God’s grace will cover all their unrepentant sins. So they think they can keep on sinning the way they do, and God will forget about it. They’re religious humanists in the church. They’re accommodation junkies. And I can assure you, God won’t forget about it.
This accommodation is a neighbor to relativism: there are no absolute standards. I’m okay and you’re okay, as long as your okay doesn’t infringe on my okay. It’s encapsulated in the slogan: “Celebrate diversity.” Apparently this celebration doesn’t include moral judgments, which are the anathema of our relativistic age, and, in fact, the only surviving acceptable moral judgment. No one is immoral except people (like us biblical Christians) who say that other people are immoral. This form of diversity is unforgivable in a culture that “celebrates diversity.” They want to celebrate diversity, but only as long as it’s sinful diversity. They’re accommodation junkies.
This is not, to put it mildly, the spirit of the great figures of the Bible. Elijah didn’t seek accommodation with the prophets of Baal. Jesus didn’t accommodate the Pharisees. Paul didn’t accommodate the Judaizers. John didn’t accommodate the Docetic heretics, who denied the humanity of Jesus Christ. The men and women of God whose holy exploits appear in the Bible worked to sharpen distinctions between good and evil, not to blur or erase those distinctions. They weren’t interested in accommodation with false religion; they were interested in fidelity to God.
 Rousas John Rushdoony, By What Standard? (Vallecito, California: Ross House, 1995), 1–7.
 Stephen B. Clark labels liberalism as “theological secularism.” It strips the Faith of the true God and replaces him with atheistic dogma. See his “Orthodox, Protestants, Roman Catholics: What Basis for Cooperation?” Summons to Faith and Renewal, Peter S. Williamson and Kevin Perrotta, eds. (Ann Arbor, Michigan: Servant Books, 1983), 90.
 J. Gresham Machen, Christianity and Liberalism (Grand Rapids: Eerdmans, 1923).
 Francis A. Schaeffer, The Great Evangelical Disaster (Westchester, Illinois: Crossway, 1984).
 Ibid., 99.
 Ibid., 146.