Before writing his famous book, Mere Christianity, C.S. Lewis was told by many advisors that ordinary Christians would not be interested in theology, that “dry old stuff,” but rather in plain, practical religion. He countered that he really didn’t think such ordinary readers were so foolish. He thought they would welcome the study of theology, which means “science of God.” “Any man who wants to think about God at all would like to have the clearest and most accurate ideas about Him which are available. You are not children: why should you be treated like children?”

He goes on to liken theology to a map. Theology is not first-hand religious experience or direct reading of the Bible, both of which are very important. Rather, Christian theology is a map based on the experiences and readings of thousands of intense and educated Christians throughout the centuries who really did experience God and read the Bible avidly. Their thinking provides a clear outline of what key teachings about God, Christ, the Holy Spirit, salvation, and the Christian life are essential to biblical Christian faith and what ideas and claims are not, including those that are genuinely mistaken. A map guides you in the proper direction and marks those departures that lead you astray from classic faith.

When he then proceeded to write Mere Christianity, Lewis did not just write any old—or new—theology. He aimed with great success “to explain and defend the belief that has been common to nearly all Christians at all times.” Further, he said, “I am not writing to expound something I could call ‘my religion,’ but to expound ‘mere’ Christianity, which is what it is and was what it was long before I was born and whether I like it or not.” In other words, he was trying to articulate the Great Tradition—those bedrock beliefs of the Bible, the early church, the creeds, the Reformers, and orthodox Christians throughout the ages.

Lewis was not oblivious to the many varieties of Christianity. He likened them to small halls and abodes that branch off from the great hall in which all Christians gather to affirm their common faith. The small halls and rooms are the places where Christians are nourished by the distinctive teachings and practices of their particular Christian tradition. There are Baptist halls, Lutheran halls, Catholic halls, and many more. But the distinctives that are celebrated in the small halls should not conflict with what is affirmed in the great hall of “mere” Christianity. They are particular interpretations of the common faith, not substitutes or competing versions of it. We are Christians first before we are Presbyterians or Methodists.

So the theological articulation of our common Christian faith—spiced with denominational distinctives—is what should provide the guidance system for our churches, whether or not those churches are shaped by hierarchical or congregational church orders. Such an articulation provides the map that allows them to move in the right direction and avoid the pitfalls that have plagued the church throughout history.

We badly need such a guidance system in our time because the Christian church in America is facing more aggressive challenges to its core beliefs than we have experienced for a long time. Two challenges are the most pressing and must be met with clear and firm guidance from the Great Tradition.

The first challenge disputes the orthodox teaching that Jesus Christ is only path to salvation, that Jesus and his work of redemption are unique and decisive for all the world. This is sharply opposed by current cultural demands that any acceptable faith be inclusive, universal, and non-judgmental. Any culturally accepted faith must affirm and include everyone as they are—the classic requirements of repentance, forgiveness, and amendment of life are simply too demanding. Further, Christians must drop their exclusive claims that Jesus is the only way and admit that all religions are simply different paths to the same goal. Evangelism must be turned into dialogue. Further, the old claim that there are two destinations for every soul—either heaven or hell—must be given up for the more palatable claim that all will be saved in the end. Thus, Christians must be tolerant of all sorts of beliefs since they finally will not matter anyway.

Now this challenge is held not only by “New Age” people outside the church who are “spiritual” but not “religious.” It is also held within churches by many theologians who are willing to compromise the teachings of the Great Tradition for those more compatible with a world that emphasizes diversity and tolerance. But the Great Tradition allows for no such distortions—Jesus is the Way, the Truth, and the Life. Once the church departs from that affirmation it will soon lose its special mission to convey the Gospel. The decline of the missionary impulse in the mainline denominations is a case in point. Their over-involvement in political agitation is another.

The second great challenge—even sharper and more intimidating than the first—is aimed at the churches’ teaching on marriage and sexual ethics. The sexual liberation themes of the 60s have worked through the culture with relentless momentum and are now attempting to render classical Christian teachings outside the bounds of respectable public discourse. A Supreme Court Justice has even suggested that holding to the traditional teachings is mean-spirited, bigoted, and irrational, no longer fit for consideration by people of good will.

Many activists and theologians within the mainline denominations have succumbed to the cultural trends and convinced themselves that their churches can marry gays and lesbians, as well as accept partnered homosexuals as pastors. Homosexual conduct has been morally legitimized as a concession to the culture. These moves have been church-dividing because they are so obviously against the plain sense reading of the Bible as well as the long tradition of Christian sexual ethics. The same denominations have accommodated their teachings of allow for pre-marital sex, co-habitation, and abortion. Their slope is as slippery as the culture’s.

But the C.S. Lewis’ reading of the Great Tradition, of the Bible, of “Mere” Christianity, does not allow for such sliding. Neither do those of theologians who do theology on behalf of orthodox churches, be they Baptist, Lutheran, Catholic, Presbyterian, Methodist, or other sorts. The Great Tradition simply doesn’t allow it. Rather, it points to the sorts of teaching that are faithful to the apostolic faith that has been handed down to us from the beginning of the church in the New Testament. That should be our guidance system.

A theological map in accordance with the Great Tradition is necessary as the guidance system of all orthodox churches. Without it they will stray and experience real ship-wreck.