Michael Horton in The Gospel Commission: Recovering God's Strategy for Making Disciples helps answer the question:
"It all comes down again to distinguishing between law and gospel. When it comes to ordering our lives, neighborhoods, states, and nations, there's a lot of agreement between Christians and non-Christians. It is arrogant to assert that Christianity is unique in its wisdom for living, that Christians as a whole live better than non-Christians as a whole, that only Christians are good friends and neighbors, and that only they really care about--or at lease have the right answers for--the pressing issues and temporal needs all around them. Some non-Christians I know have better marriages and families than many Christians. When we defend Christianity as 'the only way,' we have to be careful to first point out that, properly speaking, Christ is the only way, and that he is the only way to reconciliation with God, forgiveness of sins, the new birth, and all the blessings of our inheritance in the new creation. Non-Christians can follow good advice; they can turn over a new leaf and improve their actions and even their moral character. The Nation of Islam has a fine track record of turning gang members into upstanding citizens. What these groups, programs, religions, and therapies cannot do is transform people from enemies of God into friends, from condemned criminals into redeemed heirs, and from citizens of the kingdom of death into citizens of the kingdom of heaven. Only in Christ can we be forgiven and made new, not just better.
So, there are a lot of good things for which Christianity is not the only way. However, for that which is most important in life--to be reconciled with a holy God--Christ is indeed the only way.