What Do You Mean… “The Unpardonable Sin”?

Can a Christian commit a SIN, AND STILL BE A CHRISTIAN? Or would this be “the unpardonable sin”? Or would it prove he never was a Christian? Thousands worry, because they do not UNDERSTAND what IS the sin that shall never be forgiven. NOT SO LONG AGO, a professing Christian – a prominent churchman, of what denomination I never knew – was murdered for allegedly committing adultery.