You ask whether there is anything “equivalent” to the Islamic Shahādah or the Jewish Shema in Christianity. I can think of several essentializations of our Way, including the text you mention, John 3:16.

But if one is looking for something closer in form to these other two Semitic distillations, 1 Corinthians 8:6 would perhaps be the best choice. “To us there is but one God, the Father, of whom are all things, and we in Him; and one Lord Jesus Christ, by whom are all things, and we by Him.”

I’m tempted to say that the entire Christian faith could be worked out simply on the basis of those four prepositions: “to”, “of”, “in”, and “by”.