If certain religions have the concept of one God but have forms or manifestations of that one God that they worship, why is that not considered acceptable in Islam?
Bismillah
Thank you for your question. The problem with such religions is their specification of certain manifestations of God and then what their beliefs about those manifestations are. In Islamic mysticism the issue of manifestation has been discussed at length and an absolute lower boundary to understand manifestation is that it should not result in idolatry, which is exactly what the specification of forms to which people direct worship is. Monotheism is not only the specification of one God, but the unity and unlimited nature of God and His being above any material being.
May you always be successful.