Disclaimer: I own nothing. I profit nothing, except the enjoyment of taking some characters out to play.
Author's Note: First, this is set just after the angels fall at the end of 8x23. Second, the Biblical passage denoted as "KJV" comes from the King James Version of the Holy Bible.
"Set me as a seal upon thine heart, as a seal upon thine arm: for love is strong as death; jealousy is cruel as the grave: the coals thereof are coals of fire, which hath a most vehement flame. Many waters cannot quench love, neither can the floods drown it…" – Song of Solomon 8: 6-7, KJV
Castiel found himself sitting alone in a shaft of moonlight coming through the window of an abandoned church in the middle of he-knew-not-where, not far from where he'd fallen, an old, crumbling Bible open in his hands. He stared at the words, familiar, but also foreign, because he hadn't always understood them as he does now. One fateful assignment, pulling Dean Winchester out of Hell, changed his existence, his life, forever. It was so easy now to divide his existence into two periods: before Dean and after Dean.
Before Dean, he'd always thought of these words as metaphor for the love of his Father for His children. Most of the human theologians agreed with him.
After Dean, he knows there's more. He knows the visceral truth of these words, the very human inspiration behind them. Dean carried Castiel's mark on his shoulder and sigils on his ribs, not realizing that his mark was simultaneously seared into Castiel's heart. Touching a soul so bright that it was a beacon in the darkness of the Pit, one strong enough to defy both Heaven and Hell for the good of humankind, one able to get out of Purgatory with nothing more than a lot of determination and a little guidance, one so willing to continually sacrifice himself for others to be saved, had burned and branded him to his very core and challenged everything he thought he knew.
Before Dean, he never questioned his orders. Even if he didn't like them, he carried them out, trusting his superiors to tell him the truth, trusting that they knew his Father's will. After Dean, he would learn that, for much of that time, it wasn't his Father giving the orders, at least not directly and in real-time. He learned that, in fact, his Father had fled without him noticing. He found that he appreciated the irony that God resurrecting him, more than once, after his first rebellion was the way he knew his Father was still out there and still cared about the fate of Heaven and Earth.
Before Dean, Castiel hadn't known much of free will, thinking it wasn't really possible for his kind. After Dean, Castiel knew free will well: its power and greatness, but also the terrible consequences it often carried. He was currently suffering one of the consequences, and yet he couldn't bring himself to wish he'd never known what it was like to think and feel and decide for himself.
Before Dean, love and forgiveness were distant concepts. Angels weren't supposed to be emotional, to love anyone or anything but the Father and His orders, and Father was usually the one giving forgiveness. After Dean, love was alive, beating inside him like a human heart, and he knew what it was to both forgive and be the one in need of forgiveness, to be repeatedly forgiven even when he didn't feel he deserved it, and the one offering it to someone who felt he didn't deserve it, either.
For love, he rebelled and helped the Winchesters stop the apocalypse, which led to a civil war in Heaven, but saved the Earth. For love, he made terrible mistakes, like pulling souls and leviathan from Purgatory in an attempt to patch up the mess Heaven and Earth had become, and making deals with demons for the same reason. For love, he died, more than once. For love, he broke Naomi's mind control—the greatest expert in mind control known to him—and did not kill Dean. Instead, he healed him. For love, he attempted to seal the gates of Heaven, thinking that with the gates of Hell also sealed, the one he loved above all, except maybe his Father, would be able to know peace and have the life he wanted and deserved. For love, he found himself tricked. For love, he fell and became human. For love, he was determined to live and do whatever it may take to set things right.
Castiel could also attest to love's persistence. He remembered the first time he was told he was becoming too close to the humans in his charge, especially Dean. He tried to squash down what he was feeling. He tried to go back to being a good soldier and doing his duty without question. It didn't work. He couldn't turn away. He couldn't stand by and let Michael and Lucifer turn his Father's most treasured creation into collateral damage, couldn't let Dean become collateral damage. Mostly, he found he couldn't bring himself to disappoint Dean. Separation and death didn't rid him of love. The horrors of Purgatory didn't rid him of love. All the terrible things he'd done, all the good things he'd done, all thoughts of how far he'd strayed from what he'd been, and what he'd been taught, didn't make the love go away. Nothing he'd found so far did.
Being here, away from Dean, knowing what could be happening, or could have already happened, to Sam only intensified it and made him ache, made him wish he still had his Grace, so he could be there to heal Sam or, if he was too late for that, to pick up what would surely be Dean's broken pieces and put them together again the best anyone could. He wished he'd listened to Dean, should have trusted him, even if he couldn't bring himself to trust Naomi. He wondered if Dean would forgive him this time. At the same time, he knew it didn't matter. The love he carried, no matter if it was returned or not, whether Dean forgave him or not, would still be there.
He put the old Bible down carefully, afraid to turn any more pages lest it fall to dust, and gently closed it. He sighed. Since he couldn't just carry himself to Dean and Sam with a thought and the sound of wings, and since he couldn't, at the moment, fix what he'd broken, Castiel did the only other thing he knew how to do that could possibly help. He prayed.
"Father, I have once again failed you and harmed your creation, this time by helping Metatron cast our family from their home. If you choose not to listen to me, I understand. But, Father, I am begging you, please do not punish Sam and Dean. Please do not let Sam sacrifice himself. Dean won't recover. I fear that, if Sam dies, Dean will soon follow. I love him, Father." A sob cut his speech and the silence. "With me, do as you wish. I deserve death, but please let Sam and Dean live. Please help them heal. They have done so much for so many. They deserve some small happiness in life." He paused to wipe away now-human tears. "If you see fit, please guide me. I do not know how to fix what I have helped destroy, but I want to. I love my brothers and sisters, and you, and I am so sorry for failing you and them. I will do whatever it takes. I will die. I will suffer in whatever way necessary, just please help me fix it. I have faith and know that you are out there somewhere. I pray that you still care. Please, Father, please…" He trailed off, unable to keep speaking through his tears and sobs. He cried until he fell asleep.
Castiel awoke to bright beams of mid-morning sun shining on his face. He didn't know whether he had dreamed or not. He couldn't remember any images, but he was left with an impression, words running in a loop in his head in the assumed-dead prophet Chuck Shurley's voice: "Follow your heart home, Castiel. That is where your redemption begins."
He sat in silence for a moment, contemplating what it meant. He knew these were words from his Father, an answer to his prayer. Out of the corner of his eye, he noticed the Bible he had closed the night before was open again, to the same passage he'd read. He then knew his next step. He had to get to Kansas to the bunker, to Dean, home. He wasn't sure what he would find, or what he would say or do once there, but he was sure he had several hours or days to figure it out.
He got up, walked out the front door, and began walking down the gravel road in front of the church, following his heart home.