Few questions in life are more fundamental than these: "Can I know that I have eternal life and could I ever lose it?" Fortunately, the Bible provides clear answers to these questions and is the basis of my assurance.
Eternal Life is a Free Gift
It is very clear that eternal life is a gift. Note Jesus' statement to the Samaritan woman: "If you knew the gift of God, and who it is who says to you, 'Give Me a drink,' you would have asked Him, and He would have given you living water." He later states: Whoever drinks of this water [from the well] will thirst again, but whoever drinks of the water that I shall give him shall never thirst. This clearly indicates eternal security. For if a person could ever lose eternal life he would obviously thirst again. No amount of works (either before or after believing) can earn eternal life.
Eternal Life is Permanent
It is very evident that eternal life is permanent. For example, Jesus states: "I am the bread of life. He who comes to Me shall never hunger, and he who believes in Me shall never thirst." He also said: "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 the Father who sent Me, that of all He has given Me I should lose nothing, but should raise it up at the last day." Eternal life is a forever gift, appropriated once-and-for-all by faith. The moment I believe I am eternally secure - based on the promises of God.
Assurance of Eternal Life