Salvation is a gift, there is nothing that a person can do to earn salvation but it is simply by the grace of God that we are saved. Once a person accepts the gift of salvation it is something that can never be taken from him(or her) but as with any gift it is something that can be thrown away or returned but that is a decision to be made by the individual.
While God would want us to do as many good works as we can, there is no amount of good works that can be done to save us,as I said above, it's a gift and the only condition is to accept Jesus and ask for salvation. You can reject Jesus and your salvation anytime you please but you don't have to worry it being taken from you.