I don’t think any Christian can be 100% sure, but if he or she lives for Christ on a daily basis, follows God’s commandments, confesses his sins, loves his neighbor as himself, and loves God with all his heart, mind, and strength, I think that person can sleep well at night regarding where they stand with God.
A person deceives himself if he thinks he can sin without repentance and thinks God will approve and allow him to enter into heaven regardless. On the other hand, I think God wants to give us a “blessed hope” (Titus 2:12-13) and not have us worry and doubt our eternal destiny, either, living in fear like a scared little kitten.
I don’t think God wants His people to be paranoid and insecure regarding their eternal status, turning into scrupulous people who try to achieve salvation through legalistic means as if we could achieve salvation on our own through good works. Good works are important, but we must have faith, also.
Luke 10:25-27
And a lawyer stood up and put Him to the test, saying, “Teacher, what shall I do to inherit eternal life?” 26And He said to him, “What is written in the Law? How does it read to you?” 27And he answered, “YOU SHALL LOVE THE LORD YOUR GOD WITH ALL YOUR HEART, AND WITH ALL YOUR SOUL, AND WITH ALL YOUR STRENGTH, AND WITH ALL YOUR MIND; AND YOUR NEIGHBOR AS YOURSELF.”
When I die, I trust that I will be with Jesus, not out of arrogance or some false eternal security policy, but because I have put my trust and faith in Christ to be my Lord and Savior and have done my best to live out that faith in Him on a daily basis through my actions here on earth.
In doing so, we can be encouraged by the promises of Christ, such as in John 14:3:
"And if I go and prepare a place for you, I will come back and take you to be with me that you also may be where I am".