What is the one thing that you most want me to understand about Islam?

Well if there is one thing I want you to know about Islam and read from the Noble Quran, it would be the following Noble Verse:
"Say: 'O People of the Book (i.e., Jews and Christians)! Come to common terms as between us and you: That we worship none but Allah; that we associate no partners with Him; that we erect not, from among ourselves, Lords and patrons other than Allah.' If then they turn back, say ye: 'Bear witness that we (at least) are Muslims (bowing to Allah's Will).' (The Noble Quran, 3:64)"
Yes, I really wish for you worship none but Allah Almighty. Trinity and other forms of Polytheism such as idol worshiping are not the way to Allah Almighty. It's really ironic how trinitarian Christians refer to trinity as "The Holy Trinity". What Holy Trinity? What's so holy about it if it's not even directly defined in the Bible? The word itself (trinity) doesn't even exist in the Bible. Not even once!