Q: If there is no sin in heaven, then how was it possible for Lucifer to sin against God?

A: There is agency in heaven. Lucifer made a choice. The fact that sin cannot exist in heaven is the very reason he was cast from it.

We know that at the Last Day of Judgment, God will cause the heavens and the earth to be made anew. The last few chapters of The Revelation give a description of what the new heaven will be like. We have no way of knowing how much of that description is literal. Much of it is figurative and symbolic. But we do know that there will be no pain, no sadness, no sorrow, and no death when we get there (Rev 21:1-27) .

I can't imagine total happiness and joy, a state of bliss if you will, without the aspect of agency or free will. Without it we would be mere puppets... never making a decision of our own. So it seems likely (to me) that agency will continue to exist, even in the new heaven. However, consider this. Only Satan and a very few of his followers have ever made a decision to leave heaven once they have experienced it. This is our assurance that the ability to sin will not be an issue for us once we are there. In a state of complete peace and contentment, in a place without sin, why would one ever be tempted to sin?

In deed, why was Lucifer tempted? The Bible tells us that we will be greater than the Angels (1 Cor 6:3) , and like Jesus, we will be glorified in heaven. The Bible tells us that in heaven, we will see all things clearly (1 Cor 13:12) . Jesus was tempted to sin while here on earth (Mat 4:1-11) , but was able to see sin for what it is. Once in heaven, like Jesus, we will recognize completely the value of our relationship with God. We are not mere creations... we are children of God, made in his likeness (Eph 4:24) !

While it is true that Adam and Eve were tempted, and fell from grace, there is nothing in the Bible that says they were in heaven while in the Garden of Eden, except for the fact that God walked and talked with them while they were in their state of perfection. If the Garden of Eden was part of heaven at that time, it ceased to be so, once sin was introduced. Now the earth groans to be restored to this state of perfection (Rom 8:19-25) , but this can only occur once Satan, and everything that is evil, is at last removed from earth, as well as in heaven.

"Thy will be done, on earth as it is in heaven" (Mat 6:10) .