The origin of Unitarians as a movement is fairly instructive. The Congregationalists (descendants of Puritans, kind of) were hard-core Reformed and determinists with a very strict view of God. They ended up not believing it was possible to know whether you were a true Christian ("Elect") since God decides and is inscrutable.

Within about a generation, most Congregationalists became Unitarian Universalists, which is strongly linked to uncertainty over eternal blessedness/damnation. (Generalizing the history quite a bit, TBH.)