Assurance is entire confidence; most certain confidence; freedom from doubt and uncertainty. It is to “know” (1John 5:12-13).
True or full assurance is not a feeling. Feelings are fickle. They can waver from time to time as the result of our insecurities and fears. Assurance is not a feeling but truth based on the Word of God; the source of absolute truth.
When used in the context of our salvation, assurance and security are tied together but unique in the following way. The security of the believer is the work of God which guarantees that the gift of salvation, once received, is forever and cannot be lost. The assurance of salvation is the realization, the full confidence, that the genuine believer does possess eternal life.