Theologically, assurance is the state of being confident in a condition or outcome. Usually, it is applied to one’s assurance of salvation.  Texts often used to support assurance of salvation are John 10:28 “and I give eternal life to them, and they shall never perish, and no one shall snatch them out of My hand,” and 1 John 5:13, “These things I have written to you who believe in the name of the Son of God, in order that you may know that you have eternal life.”  This assurance is given by the Holy Spirit.  People will often doubt that their salvation for a variety of reasons, typically relating to sin they commit, it is at these times when the true believer needs assurance.  When a believer commits sin it should be followed by repentance, as one goes to the cross again for the forgiveness of sins.