One of them, a Secret Service agent named Al Whicher, a husband and father of three, had been on my presidential security detail. Nineteen children also lost their lives.

Those who survived endured terrible pain and loss. Thankfully, many of them took the advice of a woman who knew how they felt. A mother of three children whose husband had been killed on Pan Am Flight 103 in 1988 told them, “The loss you feel must not paralyze your own lives. Instead, you must try to pay tribute to your loved ones by continuing to do all the things they left undone, thus ensuring they did not die in vain.”

We are all grateful that so many of the attack's survivors have done exactly that. We must also never forget the courageous and loving response of the people and leaders of Oklahoma City and the state of Oklahoma, as well as the firefighters and others who came from all across America to help them.

In the wake of the bombing, Oklahoma City prompted Congress to approve most of the proposals I submitted to develop a stronger and more systematic approach to defending our nation and its citizens against terrorism, an effort that continues today, as we saw with President Obama's impressive international summit meeting last week to secure all sources of nuclear material that can be made into bombs.

Finally, we should never forget what drove the bombers, and how they justified their actions to themselves. They took to the ultimate extreme an idea advocated in the months and years before the bombing by an increasingly vocal minority: the belief that the greatest threat to American freedom is our government, and that public servants do not protect our freedoms, but abuse them. On that April 19, the second anniversary of the assault of the Branch Davidian compound near Waco, deeply alienated and disconnected Americans decided murder was a blow for liberty.

Americans have more freedom and broader rights than citizens of almost any other nation in the world, including the capacity to criticize their government and their elected officials.

But we do not have the right to resort to violence — or the threat of violence — when we don't get our way. Our founders constructed a system of government so that reason could prevail over fear. Oklahoma City proved once again that without the law there is no freedom.

Criticism is part of the lifeblood of democracy. No one is right all the time.

But we should remember that there is a big difference between criticizing a policy or a politician and demonizing the government that guarantees our freedoms and the public servants who enforce our laws.

We are again dealing with difficulties in a contentious, partisan time.

We are more connected than ever before, more able to spread our ideas and beliefs, our anger and fears.

Civic virtue can include harsh criticism, protest, even civil disobedience. But not violence or its advocacy. That is the bright line that protects our freedom. It has held for a long time, since President George Washington called out 13,000 troops in response to the Whiskey Rebellion.

Fifteen years ago, the line was crossed in Oklahoma City. In the current climate, with so many threats against the president, members of Congress and other public servants, we owe it to the victims of Oklahoma City, and those who survived and responded so bravely, not to cross it again.

Bill Clinton, the founder of the William J. Clinton Foundation, was the 42nd president of the United States.