In Philippines 2:12 we are challenged to "work out your salvation with fear and trembling."
The Bible teaches that salvation cannot be earned, so how can we know that we are truly followers of Christ? Pastor David Platt addresses this question about how to know whether or not we have salvation. Platt cites 2 Corinthians 13:5.
Test yourselves to see if you are in the faith; examine yourselves! Or do you not recognize this about yourselves, that Jesus Christ is in you-- unless indeed you fail the test? - 2 Corinthians 13:5 NASB
Pastor David Platt also suggests that Christians study 1 John to know if they truly belong to Jesus Christ.