People are offended that God is God.
People are offended by other peoples' misapprehension of who God is. None would be offended if they knew Who He truly is.
And no, people get offended when other people play God. When other people, pretending to be His representatives, point their collective fingers at others and proclaim them as being condemned, cultic, unconverted, un Christian. Yet Jesus came not to condemn the world, but to lay His life down for many. If God chooses some to salvation, and others to condemnation, what is that to you that you would bother to come here to preach that some here are converted, and some not? As if that changes anything, or by our agreeing with you (or disagreeing) we can alter our destiny?
What if I, being an Adventist, should of a sudden say, "Goodness! You are correct!!! I am in a cult, therefore not saved. Woe is me!!" To what purpose would such a confession avail me? Apart from etching a notch on your own belt of self righteousness, how could such a confession affect my destiny if my decisions for or against God are meaningless? Why bother??? Even if I should fall to my knees in total repentance and surrender, confessing my helplessness and poverty, my complete depravity and incapability to make even a right decision without Him, and plead for His mercy and grace, what would any of that avail me if God has already predestined me to hell? Would all that repentance, no matter how sincere and honest, if God has decided to condemn, nothing more that a human work in futility to gain salvation?
Now of course you may respond and say, that if a person has indeed done all of that, then it is because God has indeed elected him to be saved, and he therefore is saved. And his confession was simply the fruit of God's work upon his life. Right? But how do you know that there aren't people within certain churches you personally disapprove of, that have done precisely that, and God is in the very process of delivering them from those churches? Unless you somehow can prove that they are saved only once they come out, then they are Christians, despite their current allegiances are they not? After all, if God didn't have His own people, saved and secure, yet in Babylon, why would He be calling them out?