If you believe you had the ability to say yes to God in believing the gospel, how can you be sure that some time you may not decide to say no to God-even in eternity in heaven?

Zins expresses the problem as anyone:
It is ironic that many...who adamantly argue that God forces no one to come to Him have no problem believing that God forces those who have come to Him to say with Him. For most evangelicals, free will mysteriously disappears after one chooses salvation...."God will not make you come, but He will make you stay," might be their theological sentiment.
This is a mischaracterization, because we don't say God forces us after salvation, nor do we say free will mysteriously disappears. Try to understand. The answer is simple as the Bible itself: Why would I ever want to give up heaven? There would be nothing to tempt me away from the Lord, who is so wonderful that nothing could! Nothing can exceed what Jesus did on the cross for me.

But Satan was in the presence of God and rebelled! Yes, but he was never bought with the precious blood of Christ. This gratitude keeps a person from ever rejecting God. This is new birth.

Who tempted Satan though? It was his own pride. But angels are different beings. They follow a different paradigm than we do. What we can say is a fallen angel or demon will never repent. When they made the choice it was for forever and they knew that. Man gets the redemptive design once fallen to choose or reject the salvation offer because we are fleshly beings. The flesh can't be redeemed; it's verdict is death. Hence the crosswork which puts it death when we die on the cross with Christ and we receive a new life through His resurrection. In Revelation 5 the angels have no way to experience the precious blood of Christ in forgiveness of sins, but they can praise the redeemed ones.

The resurrection, delivering us from these bodies of sin, suffering and death is going to solve the problem. "The flesh lusts against the Spirit, and the Spirit against the flesh...the two are contrary, so you can't do what you would." "O wretched man that I am, who will deliver me from this body of death?" "I thank God, through Jesus Christ."

Jesus said, Thy word is turth...I am the truth...the resurrection and the life." He promised believers eternal life...that we would never perish. I believe Him. That's all you need to know...it's that simple. By not simply accepting this simplicity you are trying exalt yourself above God, get above His mind or in His mind, like one trying to figure out something he can't understand, but keeps trying anyway. You can understand to a point, but eventually, you will not be able to understand something, because only God understands all things.

Salvation nor assurance of salvation is by works, nor can works be a sign of the reality of one's salvation or the means of providing assurance. Even the apparent working of miracles, casting out of demons, and prophesying in Christ's name are no proof that one belongs to Him, as Christ solemnly declared:
Not everyone that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, yet that work iniquity. (Matt. 7.21-23).
If you read this verse as believers who shall lose reward, that is a correct rendering. But also, for those who are unsaved, no amount of work can save them.

On the other hand, there could be in the life of a particular person not one good work to indicate the reality of salvation, yet that person could be truly saved and thus elected of God to the blessings He has planned for the redeemed for all ages.
Every man's work shall be...revealed by fire; and the fire shall try every man's work of what sort it is. If any man's work abide...he shall receive a reward. If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire. (1 Cor. 3.11-15)
This is the fire of burning off false works and the loss suffered in outer darkness (no fire or furnace about it) shall be without the reward. If being elect without having the choice is the basis for assurance in salvation, then there can be no assurance, because nobody could no for certainty they were selected. But Christians do know we were selected because we fulfilled the condition for believing and receiving Christ.