Here are a few thoughts on why I think Protestants don’t like theosis as a soteriological category. These are a few claims that I’ve heard about why it’s not good. Some are more difficult to overcome than others.
1) It is unfamiliar.
2) It unseats justification: As a competing soteriology, it impinges the sole place to be held by justification, especially because justification language isn’t really used at all to describe theosis.
3) It is synergism: Not only does it replace justification, but it smacks of a works-based salvation.
4) It is idolatry: It confuses the created with Creator. That is, it repeats the sin of Adam and Eve.
5) It is mysticism: Closely related to #4, is the idea of fusion of the created with the creator (a la neo-platonism). Whereas #4 is ideological, #5 relates to ontology.
6) It sounds like Mormonism.