The verse in question is 1 John 5:18. I will put in a couple different translations to show how tricky this particular verse is.
So, for all of us who truly believe we are saved, is this verse true for you? Anyone who can honestly say they haven't sinned since they got saved is naive and deceiving themselves (1 John 1:8). But doesn't this verse say that if we do sin after conversion we weren't really saved? Ask anyone who has been in habitual sin after conversion and they'll tell you they wrestled with their own salvation. I know I have when I've been in that position.
So let's examine this verse and see if we have to redefine what it means to be saved. I once heard a street preacher say that he hadn't sinned since he'd gotten saved and that if any of us who call ourselves Christian commits any sin, we're not truly saved. When asked what grace is, his response was "grace means not sinning anymore." Not quite sure about all that, and if Brother Jed/Jim/Bob/etc ever comes to your campus, just ignore him. Was the fact that I struggled with sin (and still do) evidence that I was not truly saved?
I would reference us back to chapter 3, in particular verses 6-7 and 9-10. The continuing theme here is abiding. Abiding in Christ means living through Him and being in fellowship with Him. It's best described by us now as our "Walk" with Jesus. The concept of abiding in Christ is contrasted with "abiding in sin." Abiding is where you live, who your master is, etc. Those who abide in Christ will sin, but sin is not their master. They will come to a point of repentance and guilt over their sin. Those who abide in sin won't have that necessarily, because Christ is not their master and He does not dwell in them.
The heart of this verse in question is not "If you sin you're off the team" but rather a call for an examination of the heart. In the preceding verses John lays out the mandate for Christians to call each other to account in terms of sin. For the good of the body of Christ and the soul of the individual it is necessary for us to lovingly and humbly confront our brothers and sisters regarding their sin. It is with that action taking place first that John puts in verse 18. The one who truly abides in Christ will not continue in habitual sin (either when confronted or on their own dealings with God in repentance).
Brothers and sisters, the beauty in this verse is that as a Christian, your chains have been released and you've been set free. You have life that is free from guilt, shame, and pain in Jesus. It is a call to higher living, holy living, not a scorecard to see how good or bad you are. Trust Him to set you free, and call to Him for forgiveness, mercy and grace.