There are numerous ways in which Christianity can be understood to be offensive. One could point to the "blasphemous" claims of Jesus that he was, and is, God; or to the foolishness of believing in the resurrection; or the judgment cast, and violence committed by, many in the name of Christ; or the hypocrisy of its practitioners; or simply its claims upon truth.
Yes, there are many ways in which Christianity can be understood to be offensive.
Yet, there is an offensiveness stemming from Christianity that many Christians are (perhaps) oblivious to, but of which non-Christians are keenly aware. What is this understated offense of the Christian faith? The sovereignty of God and lordship of Jesus Christ.
What I mean by this is that the Triune God is at the center of our faith. This God created and sustains all things (BC, Art. 12), leads and governs all things according to his will in the outworking of his plans (BC, Art. 13), and is the central actor in bringing about the redemption of humanity and the world.
Furthermore, the lordship of Christ gives further definition to the work of this Triune God, as all things belong to him. Rather than by power and through the use of weapons, Christ is at work advancing his kingdom through Word and Spirit. Those who trust in Christ recognize him as their king; a king who has authority over, and actively governs us, as well as guards and keeps us in the freedom he has won for us from the powers of sin, death, and the evil one (HC, Q&A 31).
And while Christians may give a hearty "Amen!" to these words, they serve as yet another stumbling block to those around us. The reason? Such claims are in conflict with human autonomy.
In his well known work on applying utilitarianism to various aspects of life, On Liberty, philosopher John Stuart Mill makes a claim that has since taken root in our culture: Over himself, over his own body and mind, the individual is sovereign. In such a system, the authority is invested in human institutions, communities, and the individual, and the truth is found within. We are captains of our ships, and we determine which ways we will go.
So it is no wonder that many, especially young people, find Christianity to be offensive, and reject it out of hand. It is a direct challenge on their autonomy--their self-determining, define, and directing world. For the non-Christian, to trust in Christ and reconcile with God is not merely an acknowledgement of his or her sin and disobedience to God, but even more so, it is a call to give up everything they have been told belongs to them. Confessing the Triune God and the lordship of Christ means acknowledging, and submitting to, his authority, as well as seeking to conform to his pattern for life and human flourishing (his calling for us).
The challenge for us, as Christians, is to recognize this offense (this is crucial as the lie of human autonomy has crept into segments of the church subverting its teaching on objective truth, ethics, sexuality, etc.), engaging lovingly and dealing patiently with those around us who we yearn to see enter the Kingdom, and praying fervently that God would soften their hearts and draw them to himself in faith.