Leaders provide vision. They help people understand where they are, how they got there and what they must do to go forward. They help calm nerves and strengthen courage. They are steady in times of trouble, inspiring in times of demoralization.

Donald Trump’s reaction to the terrorist acts in Egypt, Lebanon, Paris and San Bernardino, Calif., both divides and weakens us. And for the most part, his rivals for the Republican presidential nomination have allowed him to lead the Republican Party and its run to the White House to ignominy.

Americans are understandably worried. We have been fighting wars in the Middle East for over a decade. We lost thousands of lives and spent literally trillions of dollars in a wrong-headed war of choice in Iraq. We toppled the Taliban in Afghanistan, Saddam Hussein in Iraq, Muammar Qaddafi in Libya, producing failed states and generating more terrorists. Now we are trying to take out Assad in Syria even as his mortal enemy ISIS takes credit for terrorist horrors in Paris and Beirut. The violence keeps spreading; the terrorists keep reviving. And Americans grow more and more worried.

The task of leadership is to provide clarity, and a strategy that quells fears and offers a way out. Mr. Trump has chosen instead to foster greater fear, to divide us one from another, while calling on us to abandon our own values.

It is time for people of conscience to once more stand up. We are not in a war with our brothers and sisters of the Muslim faith. They are neighbors and friends. Some volunteer to defend this country. American Muslims despise ISIS and the way the terrorists distort and disgrace their religion. We should not lock our doors or close our hearts to those who are the victims of ISIS and the terror in Syria and Iraq whatever their religion or tribe. The threat to lock out Muslims is harmful, hurtful and divisive.

It is immoral and perversely counterproductive. There are more Muslims in the world than Christians. There are more Muslims than Americans. ISIS may declare war on Western civilization, but that just illustrates their madness. We are not at war with Islam. We are at war with terrorists who threaten our people. We want to enlist the overwhelming majority of Muslims to stand with us, not provoke them to stand against us.

In this Christmas season, we should not forget that after his birth in a manger, Jesus was a refugee, fleeing into Egypt with his parents from the threat of Roman violence. He taught us to care for the stranger on the Jericho Road, not spurn and scorn him.

I grew up under segregation, locked out because of the color of my skin. The race test was hurtful because I could never pass it. The same is true of the call to exclude all Muslims. We must speak forcefully against those who would mindlessly impose a religion test for entry into the U.S., and those who would call for closing down Mosques or cracking down on places where Muslims gather peacefully. America will not defeat terrorism by dividing one from another.

Terror is the weapon of the weak. For a people with hope, the hate of ISIS has no attraction. For people who are integrated into a society, ISIS’ call to violence goes unanswered. ISIS calls for jihad, arguing that the West is waging war on Islam. We need to isolate the terrorists, not inflate them. We need to give lie to their calumnies, not provide evidence for them.

Anyone who aspires to lead this country ought to demonstrate that they will respect and defend the values on which it has been built. In this time of trouble, Mr. Trump and many of his rivals are denigrating the very principles that make America strong.