I thought about something else. I hope no-one else claimed this yet. I also want to discard the theory used prior to this one.
More than 3000 years ago the clans were all locked in a stalemate in the struggle for magic power, the goddesses and demons especially were at each other's throats. Meliodas as the leader of the Ten Commandments made his way further into the enemy lines than anyone else from his clan. Time and time again he would clash with their most powerful agent - the apostle of the goddess clan. Unable to defeat each other, it became a struggle for morality which the demonic side of Meliodas was slowly but surely losing. He recognized the futility in a life full of warring and bloodshed. It eventually led to him defecting and falling for the apostle of the goddesses.
As she showed him a different meaning to life more and more throughout their encounters, she developed mutual feelings for him, first it happened in secret, later more open, cause rumors, whispers of doubts among their respective clans that their most powerful generals had... changed.
Events transpired that left Meliodas unable to carry on. He deserted, killing two of his Commandments in the process and the events triggered the decree of a third one who also became lost to the demon clan in the process - Gowther. Now severely weakened, the goddess clan saw its chance to kill the demons one and for all. But their plan failed because in the final stages of its execution, their own trump card, the apostle, had also deserted out of love for Meliodas.
Meliodas who had decided to put an end to the war once and for all fought against both goddesses and demons alike, later making friends with Dolor and Gloxinia, the respective leaders of their clans. As time went on, both Meliodas and the apostle however found less and less meaning in war. Their love was too strong for each other and looking to break away from this crazy world, they once again deserted together, leaving even Meliodas' closest friends, who then became victims of great calamities, leaving them with broken spirits.
That was the moment when the goddesses once again gathered their strengths and thought of another plan to finish the demons, this time one and for all. They convinced the other clans to join forces with them and destroy the demon realm. But Dolor and Gloxinia who had been betrayed and crushed themselves, disagreed with these underhanded tactics, believing the goddess clan was only using the other clans to rise to power themselves so they could dominate the other remaining clans with their doctrine. That and a personal loss and defeat as well as promises made by the demon king convinced them to join the ranks of the Commandments and replace two of the lost members.
Now it was the fairy clan and the giant clan left without their most powerful leaders, unable to lend their full strength to the goddesses. Despite all efforts and fighting tooth and nail, the goddess clan and the other clans only managed to drive the demons back. They destroyed their world but were unable to kill them all and were forced to seal them instead. The world seemed to be lingering in a strange void of peace. But it was a treacherous one. The goddess clan's doctrine is absolute. They do not forgive nor do they forget. And eventually they found their former apostle who had abandoned them and sentenced her to death, leaving Meliodas who had lost his Commandment's decree and power unable to do anything. Devastated by his loss, he wandered the world, unable to feel anything, to care for anything... until he became a knight in a kingdom named Danafor. The rest... is history.
