Is Christianity the only true religion?
Religion is a complex matter. When used as a means to promote self-justification, war-mongering or prejudice, it is a form of sin. Too often all religions--and not least Christianity--have been twisted in this way. Nevertheless, by grace, despite all disobedience, Christianity offers the truth of the gospel. Although other religions may enshrine various truths, no other can or does affirm the name of Jesus Christ as the hope of the world.
How will God deal with the followers of other religions?
God has made salvation available to all human beings through Jesus Christ, crucified and risen. How God will deal with those who do not know or follow Christ, but who follow another tradition, we cannot finally say. We can say, however, that God is gracious and merciful, and that God will not deal with people in any other way than we see in Jesus Christ, who came as the Savior of the world.
How should I treat non-Christians and people of other religions?
As much as I can, I should meet friendship with friendship, hostility with kindness, generosity with gratitude, persecution with forbearance, truth with agreement, and error with truth. I should express my faith with humility and devotion as the occasion requires, whether silently or openly, boldly or meekly, by word or by deed. I should avoid compromising the truth on the one hand and being narrow-minded on the other. In short, I should always welcome and accept these others in a way that honors and reflects the Lord's welcome and acceptance of me.