It is perhaps too simplistic to simply say that Stephen was stoned because he was a Christian. Two passages may provide a more specific reason. First, he put the Jews of Cyrene to shame because of "the wisdom the Spirit gave him."

Opposition arose, however, from members of the Synagogue of the Freedmen (as it was called)—Jews of Cyrene and Alexandria as well as the provinces of Cilicia and Asia—who began to argue with Stephen.  But they could not stand up against the wisdom the Spirit gave him as he spoke. (Acts 6:9, 10 NIV)

They "could not stand up against" such wisdom. This may be compared to the religious rulers trying to stump Jesus but failing to do so. Second, Stephen did not mince words when he exposed the stubborn unbelief of the Jewish people:

“You stiff-necked people! Your hearts and ears are still uncircumcised. You are just like your ancestors: You always resist the Holy Spirit!  Was there ever a prophet your ancestors did not persecute? They even killed those who predicted the coming of the Righteous One. And now you have betrayed and murdered him—  you who have received the law that was given through angels but have not obeyed it.” (Acts 7:51-53 NIV)

We are told that after these words, the people got so angry with Stephen that they stoned him to death. We celebrate Stephen to this day as the first Christian martyr. 

So, he was stoned because of his strong words. While Stephen's message in Acts 7 is the longest recorded in Acts, there is very little in the way of God's love and mercy in his gospel presentation. His was a message of accusation and a demand for repentance. I daresay many, who teach evangelism today, if this account was not in the Scriptures, would criticize Stephen's approach even while admiring his courage and faith. 

But perhaps we ought to reexamine our "friendship evangelism", "just preach the love of God" approach to evangelism today.