The Bible teaches both Assurance, and Eternal Security. Although these two doctrines both similar and related, they are conceptually distinct.
Eternal Security teaches that the believer, one having entered into a relationship with God, cannot possibly sever that relationship. Those who deny Eternal Security typicaly teach that man's works are necessary to "stay saved."
In contrast, the doctrine of Assurance states that the believer can know with absolute certainty that he is saved.
Those who deny the doctrine of assurance contend that a person who believes on Jesus Christ as their God and Savior must wait to see
if they have sufficient works of righteousness to warrant the confidence that they are truly saved. When Assurance is based on faith in Christ, it does not require works. When assurance requires works, Salvation is reduced to a system of human righteousness.
To better understand the distinctions, it is instructive to review two common theological systems that teach salvation by the works of fallen man.
One teaches eternal security, but requires human works for assurance. The other teaches assurance, but requires man's works to be eternally secure.