Fifty million people live in the Northeast I-95 corridor—one out of every six Americans. Washington, Philadelphia, New York, and Boston incubate economic might, urban poverty, old wealth, suburban sprawl, and some of the oldest and most influential educational institutions in the country.

As a teacher in the Philadelphia public schools, I am a part of the megalopolis. For those who have never experienced my city, it is difficult to express the size of the Philadelphia school district. The district educates 200,000 students. The yearly budget is measured in the low billions of dollars. Only about 100 American cities are larger than the student population of my school district and, according to Forbes, only 200 private U.S. companies have yearly revenues in excess of the yearly expenditures of my school district. I serve as a math and science teacher to students between the ages of 12 and 18.

How can a believer in Christ teach in this place? That is a question I have discussed with quite a few Christians over the years. My first response is to tell about a question I’ve gotten many times from my secular students: How can a science teacher believe in God?

Many of my students are disconnected completely from religion. They see Christian belief as an irrelevant and scary throwback to an era of witch burning and gullibility. To meet somebody who has not burned witches and who loves both science and God is remarkable to them. We in teaching like to call this sort of thing a “discrepant event”—an observation that doesn’t fit previous knowledge.

One of my fears is that an influentially large subset of my 50 million neighbors in the wealthy and powerful Northeast will never know what a Bible-reading, Christ-believing Christian is. So I show up to work and do my duty, instructing my students in the way of Galileo and Newton, but alluding to Moses and Augustine when appropriate. (For instance, if you want to understand humans and time, I say you need to consult Moses in Psalm 90 and Augustine in the Confessions. Physicists haven’t shed much light on the meaning of time, only on the mechanics of it.)

Charles Murray wrote a provocative book in 2012 called Coming Apart. In it, he explains how the economic and political elite were clustered together and seldom had friendships or conversations that crossed the “bubble boundary.” When non-church-attenders hang out only with others of the same description, they get the impression that their view is the only view. They are also completely unprotected from the standard non-Christian propaganda of the typical public school curriculum. At best, what they will know of the Puritans is the madness of the Salem witch trials and Nathaniel Hawthorne’s slanderous Scarlet Letter. Knowing a Christian personally, especially a Calvinist, gives them a chance at a different view.

In like manner, it is also good for believers to engage in significant ways with secular people. Christians who cluster together for their daily affairs and mingle only with their own kind are in danger. There are many bubbles in the world, and the evangelical Christian bubble is one of them. The Amish solution—to shun the world—should not be our solution. Jeremiah reminded the exiles in Babylon to build houses, plant gardens, and continue doing the ordinary things of life while in a strange place. Public schools are a strange place. American culture, too, is a strange place for many of the same reasons.

Some object by saying schools are indoctrination centers dedicated to a progressive model of education that is incompatible with faith. At times, I wish that schools—any schools—were that effective. Yet I’m thankful they aren’t. A school is only a part of the influence on the life and beliefs of a student. The variety of influences a student experiences in the course of a day is almost as varied as the backgrounds of the students themselves. For every English teacher spouting nihilistic absurdism, there is a math teacher encouraging an orderly, faithful life. For every science teacher who insists that the universe is a cold, impersonal, godless lump of stuff, you will find a history teacher who brings the story of Christendom to light. I do run into people who would make schools into centers of secular indoctrination. But it is part of my job (not that it’s in my official contract) to work in my own particular way to give a fuller and more interesting education than might be prescribed in the curriculum. Christ is shown through the lives of those who believe in Him.

Many Christians will point out that teaching is not spiritually neutral. Knowledge is always imparted from a set of presuppositions, and public schools do not teach from a Christian worldview. These things are true. But ever-changing, popular, secular, liberal perspectives are hardly worthy contenders for replacing historical Christian views. Schools—parents, students, teachers, and administrators—tend to validate and reflect the culture that surrounds them (the culture that everyone sees on TV and on web sites and in political campaigns), rather than follow one particular dogma. Popular and political culture changes with the seasons. Recall that Hillary Clinton supported the Defense of Marriage Act in 1996. In 2016, she could hardly stand to be reminded that she supported such a thing. This cultural change, radical as it is, has only trivially affected the day-to-day operation of schools.

The actual teaching that goes on in a public school setting owes much more to the individual teacher and much less to any policy that may be in a folder in the administrator’s office. Schoolteachers can’t hope to keep up with changing cultural fads. And the way individual subjects are taught changes slowly. Schools are, fundamentally, conservative institutions in the sense that they operate in a straightforward way according to long-settled (but flexible) patterns.

My freedom to operate within this framework is considerable. Though some administrators have tried to script teaching and learning, teachers are quite adept at resisting this incursion into their freedom. Students do not receive only one perspective when they attend a public school. By high school, students become savvy enough to know that each teacher brings their own particular spin to a subject.

Public schools educate an unbelievably varied set of students. One local school district has counted over 100 different languages spoken at home. I have had the delight of meeting a number of Albanian families (before teaching at my present school, I would have been hard-pressed to find their country on a map). Successfully teaching such varied people hinges on a high tolerance for differences. In my experience, this tolerance extends to just about anyone who will teach—even those, such as myself, who have significant religious beliefs. Within this polyglot reality, there is room to discover a shared Christian faith among students and teachers. One teacher I know, a Christian, discovered some devastating news about his unborn child. When he revealed this to his class, one student immediately tweeted the prayer request to her prayer group. When my home was destroyed by fire, I was not entirely surprised to hear from some of my students, “I’m praying for you.” I haven’t gone out of my way to identify myself as a Christian, but students usually figure it out. Some are puzzled, but more than a small number are church attenders themselves. In fact, considering the number of students in the country who attend public schools (some 50 million in the U.S.), there are many more church-attending young people at public schools than in private education or in a homeschool environment. I hope that I, and others like me, provide some small measure of encouragement to their faith.

There are some policies set by my school that would make it impossible for me to continue working, were they fully implemented. But common-sense realism and tolerance for difference are much more the order of the day than an outsider might expect. Schools must operate with order, and this means that policies about transgender bathroom access, for instance, are mostly ignored in practice even though they are written so forcefully. Long may this tolerance continue.

If there is one religious idea that I would like to communicate to my students, it is a sense of reverence for the right things. I am reverent toward God (don’t take God’s name in vain in my class, please), but irreverent about politics, self-esteem, bureaucratic silliness, and educational fads. I am also not reverent about schools, as much as I appreciate their place in society. I will occasionally throw a little fit about what they get wrong. (There is little that students find more entertaining than a good rant from a teacher.) Schools try to teach about a few things that they really ought to leave alone, because they screw them up so badly.

Take sex, for instance. Despite the truly astonishing array of anatomical precision that is displayed to students, they receive no true education about lasting relationships. My high school students know far too much about contraceptive methods, but almost nothing about how to become friends with the opposite sex. They know how to draw diagrams of ovaries, but they don’t know the first thing about how to become intimate with the head and the heart. My heart goes out to these sheep without a shepherd.

I have several mini-sermons I give on this subject. One of them is on my “quantum theory of love.” It is a goofy title for a simple idea: there are many people who are possible mates, but only one of them can be your choice. It’s an anti-romantic, pro-monogamy theme. A theme that says that people don’t have to wait their entire lives to find that one soul mate, but once they find one husband or wife, that is husband or wife for life. It’s a little on the silly side, so I was stunned when one young lady gushed to me after hearing it, “Oh, you’ve given me such hope!”

Christians have even more surprises than simply offering up hope of romantic love without betrayal. We bring God’s love into the equation. Students respond surprisingly well to my culturally backward approach, despite the fact that, as Richard Swinburne has written in a recent essay, “Traditional Christian teaching on many moral issues, but in particular on sex, family, and life is regarded by all non-religious and some religious believers as totally and evidently mistaken.” We may be viewed as mistaken, but the promise that we hold—that God can love us and humans can love each other with selfless devotion—is truly attractive. Love of man and knowledge of self begin with knowledge of God.

Teaching has given me an opportunity to interact with people from amazingly varied ethnic and social backgrounds. The world comes to the U.S. and goes to its public schools. Public school teachers have the opportunity to remind students of the pervasive influence of Christian thinking in American heritage and identity so that the students know there is an alternative to whatever else schools, communications media, and even their families may be teaching them.