How does one know that they have salvation? Assurance, the human being's confidence and conviction that their sins have been paid, can be best understood by first examining the basis of assurance: the concept of eternal security.

The concept that Believers can never lose their salvation, eternal security, can be seen in the words of Jesus who states that all who believe in Jesus will have eternal life:

"He who believes in the Son has eternal life; but he who does not obey the Son will not see life, but the wrath of God abides on him." (John 3:36)

"Truly, truly, I say to you, he who hears My word, and believes Him who sent Me, has eternal life, and does not come into judgment, but has passed out of death into life." (John 5:24)

The apostles attest to the same message:

These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life. (1 John 5:13)

Because salvation is based on Jesus' work of atonement (see What does "It is finished" mean? A word study of John 19:30), there is nothing a human being can do to earn salvation; eternal security is a concept that recognizes the complete and irrevocable work of God.

The concept of eternal security also recognizes the entitlement of salvation namely of becoming God's holy people and His possession (Ex 19:4-6; Deut 7:6). The idea that Believers are God's own possession is seen in Jesus' statement about the eternal security of the Old Testament Believer (see What does it mean "God draws"?):

All that the Father gives Me will come to Me, and the one who comes to Me I will certainly not cast out. For I have come down from heaven, not to do My own will, but the will of Him who sent Me. This is the will of Him who sent Me, that of all that He has given Me I lose nothing, but raise it up on the last day. For this is the will of My Father, that everyone who beholds the Son and believes in Him will have eternal life, and I Myself will raise him up on the last day." (John 6:37-40)

But you do not believe because you are not of My sheep. My sheep hear My voice, and I know them, and they follow Me and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand. My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of the Father's hand. I and the Father are one." (John 10:26-30)

This idea of holy people being God's own possession is further developed in the New Testament with the sealing of genuine Christians by the Holy Spirit at initial salvation; God's pledge and guarantee to the Believer of his inheritance.

The concept of eternal security is objective fact. Assurance is Jesus' guarantee of that fact.