"For he who is least among you all--he is the greatest." (Luke 9:48)
Once we become saved by the blood of Jesus, we are to remain in Jesus Christ.
If anyone does not remain in me, he is like a branch that is thrown away and withers; such branches are picked up, thrown into the fire and burned. (John 15:6)
This passage shows that those who are in Jesus will be saved. It also shows that someone, who was once in Christ, can stop remaining in Christ which brings the consequence of eternal punishment.
How can we be sure in we are in Christ?
This is how we know we are in him: 6Whoever claims to live in him must walk as Jesus did. (1 John 2:6)
We can be sure that we are in Jesus if we walk as he did. How did Jesus walk? He walked a life of obedience to the Father even to his death.
Simple to say, but hard to carry out. Perhaps that is why few travel on the narrow path that leads to life, and many travel on the road that leads to destruction.