Little Mary this is a great question. As soon as I read it I was drawn back to the years I spent in Freemasonry, where the question is propounded to the initiate “What makes you Mason?” and the answer is, “My Obligation”.
I believe the same holds true in Christianity. Now I am not talking about the vow that Catholic converts take to believe all that the Catholic Church teaches. On the contrary, what I am saying it that what makes me a Christian is my free choice to be obliged to follow Christ to Calvary and beyond. And God not only smiles upon that free choice of mine, but He also empowers me in it; that is to say, that He showers me with every grace to be all that He created me to be.
That is what it means to me to be a Christian - my free obligation. Thanks for the question!