As a confessional Lutheran my beliefs have boundaries as spelled out in the A.D. 1530 Confession of Augsburg:
Article I: Of God.
1] Our Churches, with common consent, do teach that the decree of the Council of Nicaea concerning the Unity of the Divine Essence and concerning the Three Persons, is true and to be believed without any doubting; 2] that is to say, there is one Divine Essence which is called and which is God: eternal, without body, without parts, of infinite power, wisdom, and goodness, the Maker and Preserver of all things, visible and invisible; and 3] yet there are three Persons, of the same essence and power, who also are coeternal, the Father, the Son, and the Holy Ghost. And the term "person" 4] they use as the Fathers have used it, to signify, not a part or quality in another, but that which subsists of itself.
5] They condemn all heresies which have sprung up against this article, as the Manichaeans, who assumed two principles, one Good and the other Evil: also the Valentinians, Arians, Eunomians, Mohammedans, and all such. 6] They condemn also the Samosatenes, old and new, who, contending that there is but one Person, sophistically and impiously argue that the Word and the Holy Ghost are not distinct Persons, but that "Word" signifies a spoken word, and "Spirit" signifies motion created in things.
Whenever an angel came to someone in the Bible, the FIRST thing the angel did was to tell the person not to be afraid and that lifted any fear. However, Muhammad was terrified of the "angel" who visited him and remained so for days after, the logical conclusion is that he was visited by Satan.