Predestination and freewill are NOT mutually exclusive truths. We have to agree upon the definition of these two terms before we could ever debate their relationship to one another. Most do not agree on what they mean so coming to a common understanding is impossible.
Even so, allow me try to explain it in this way. When God spoke, “Let it be,” with His ultimate purpose in mind, and under the umbrella of His sovereignty, He allowed creation to begin unfolding in spectacular dynamic fashion. He hand crafted everything in the boundless vaults of His fathomless mind. We were fearfully and wonderfully made.
From God’s perspective His will has been done, is being fulfilled, and is yet to come, all at the same time. Being outside of time, a limiting construct of His own design, He saw every thread as it would weave its gift of potential within this tapestry of life. The outcome was written, the fulfillment of His purpose, but not the journey. Like a brilliant fork of lightning He set the spark in motion and it flashed through the air cutting it’s own path to the ground. Each turn and every fork at the whim of the spark and yet not unknown by the One who initiated its journey.
God is never surprised by what we choose. He prepares a path before us and arms us with every tool to maximize our potential and let’s us loose. Just because He knows how it will play out, and has keyed up players who have partnered with His purpose doesn’t make it any less a free choice of ours to make. The Heavenly Father sees where our choices will take us, and within that path has predestined (predicted, a.k.a has foreknowledge of) our outcome. Like a parent who sees the great talent and potential of their child and clears a path and empowers and encourages them to succeed and reach their goal, so is the Father with us. This is the essence of predestination.
He is the Alpha and the Omega, the Aleph and the Tav, the beginning and the end. All things will work out, regardless of their choices, to the ultimate purpose of God, what we as freewill beings get to decide is to join with Him, partners in His purpose or fight against Him as enemies of His design. Even evil, which God did not choose nor create, is ultimately making God’s point, refining what is His and sifting out what is not. As beings immersed in the linear flow of time, we can hardly define these infinite concepts let alone grasp the breadth of their meaning. It is enough that we trust the One Who does and sustains all of His creation in the palm of His hand. We should focus on what He has commissioned His children to do and that is to love their neighbor, imitate their Father and disciple others in His Kingdom.