The basic premise of the story is reincarnation. The Soul Passers are essences of beings that go from one life to another, guiding the "vessel" through its life. Sometimes the essences are alone but other times there are several, the oldest (having been through many lives) guiding the younger and newer souls.
This story starts out in the middle ages in Europe. Danika has 2 souls within her and they are trying to help guide her out of a bad life and into a more enlightened one. The elder soul only offers advice to the younger, who is more than eager to help this vessel. In time Danika makes her life better through the advice of her souls.
Upon her death the souls pass on to an area where they await the next vessel chosen for them. The 2 that were in Danika separate to other vessels, learning and teaching younger souls.
1000 years later, in present day, we meet 2 different people who have strong spirits. Later in life they meet up and there's an instant connection. They know each other somehow. What their vessels don't realize is that each of them has one of the 2 souls that was in Danika. The souls recognize each other but confuse the vessels they occupy. These 2 people begin to explore each other, drawn for reasons they don't understand. But because they live far apart and only see each other once in awhile for business they don't have much of a chance to explore at first. The souls within each of them have brought them to this point, guided them to be open to meeting other souls they once knew. And in time these 2 do recognize the souls within each of them and come to terms with it.
Each of these souls are ancient. As such, they are chosen to occupy special vessels, people who can bring a change into the lives of many through their actions via the wisdom of their souls. This isn't to say that they are great leaders or prophets but are ordinary people who do extraordinary things at times, things that influence others for the better. And in their own small way help to make the world better. It's because they are born with ancient, wise souls that they are able to impart wisdom to others.
I am not well-versed in Medieval history and am learning as I go along. But if it feels uncomfortable this story could be set as a fantasy, with many parallels to our own reality.
And there will be many changes along the way. I've learned that once I start a story it takes on its own life. I feel as if I'm trying to take a rambunctious puppy for a walk but it's leading me. I'm just hanging on to the leash, letting the puppy pull me along. And the whole time I'm just taking notes and recording the event.