From Proto-Indo-European, the t sound sometimes changed to th in Proto-Germanic, which in turn gave English the same th sound. However, I'm not sure when this change happened. I watched a video at https://www.youtube.com/watch?v=5ky7SjFRsFs and it showed a th sound. So, when did the sound change happen? (It seems to have happened before ~750 BCE because that is when Pre-Proto-Germanic was spoken.)
Extra: Could the sound change happen in the opposite direction? What I mean by this is the th sound changing to t again in part because of the sound and spelling. (It has already happened in some Germanic languages, and an example outside of Germanic is the th in greek changing to t in spanish)