Father Theodore is a character shrouded in mystery. As the description under the video says, revealing any detail about his history would lead into major spoiler territory.
Of all the characters in The Evil Within 2, Theodore is perhaps the most shrouded in mystery. Any detail about his past pushes into HUGE SPOILER territory. In other words, you’ll have to play the game in order to unlock the secrets he hides. In Theodore’s mind, Mobius’ goals with STEM are misguided. They want to use the device as a means of controlling the populace, but Theodore believes he’s much better suited for the job than they could ever be. He sees STEM as a method for expanding his own influence, and seeks to circumvent Mobius from within their own device.