It's possible to be sure you're going to heaven but to end up, um, somewhere else.
But that tragedy can be avoided by heeding the words of Jesus in the Sermon on the Mount. In the last 15 verses of His Sermon (Matthew 7:13-27), Jesus tells us how to make sure we are on the narrow road that leads to life rather than the broad road that leads to destruction.
Even if you think you don't need a spiritual reality check, I hope you'll submit to this one. Too much is at stake to assume you are safe rather than putting your faith to the test.
