Adam and Eve, with their free will, but without concupiscience, committed the first sin. I presume that saints in heaven will likewise be free of concupiscience and that our (I say "our" as an expression of hope) free will shall be intact in the next life.
What is to prevent sin from happening again in Heaven?
RI:
Glory be to Jesus Christ!!
I don�t understand your inclusion of concupiscence in the question. If you mean �ardent desire,� other than its usual meaning of �ardent sexual desire,� I think we will have an �ardent desire� if the idea is that our love for God�the love that Jesus spoke of as the first and greatest commandment will know no bounds and will have no distractions of any kind.
�Sin,� as I have come to understand it is an acronym for �separation (or self-centeredness or selfishness) is normal (natural or needed). We won't be separated from God in Heaven, because Heaven is not only a place but a state of being--a state of perfect communion with God for all eternity. It seems to me that it is impossible to be in perfect relationship and be separated at the same time.
I go back to idea that this board discussed in a thread about whether we believe in a real Heaven and a real Hell. Our God is described as a �Living Fire,� that is, a self-generating love that wells up in our innermost being when we are in communion with Him. He warms the heart and moves us to love as He loves. Heaven is the state of being wherein He loves us from within, deep in our hearts, where He desires to live , even now. Hell is the embrace of that Divine Love that cannot penetrate our refusal to open the door of the heart to Him. Heaven starts now and continues after physical death. That Living Fire is already in us and we only expel Him now because we still have distractions and illusory tempttations that pretend to be as good as or better than Him. There they will all be gone.
It seems to me that your question is an oxymoron from an Eastern point of view. How can we be separated from God Who is living within us perfectly in Heaven--the place and state of being? How can we be separated from Him when in that place and state of being (known as Heaven) where our whole being is on fire with love for Him and wherein we live as the dikiri carried by the bishop�two flames touching (being in a perfect communion) but still retaining their distinctiveness?
Our free will, will be, in that place, as the perfect Will of Christ�perfectly in communion with that of the Father without distractions and without the desire to be in any other way of being.
Beyond that, Luke 12:32 tells us that it "please(s) the Father to give us the Kingdom." And doesn't Jesus tell us that the Kingdom is within? When you pose the question as you do, it seems that Western concepts of putting eveything in little pigeon hole boxes is at work. "Guarantee" seems to imply that trust is lacking in the relationship because we usually ask for this type of commitment when we don't fully trust. Do we need a guarantee from One Who wants to be "father" to us and give us so much?
Bob