As for Christianity gaining members as a result of force or violence, that began to happen in medieval times once Christianity became corrupt. As you only need to look into history to become aware of the deep perversions that became rooted in the ROman church, but if you go back to ancient history and that is what early Christianity is, ancient history, not medieval history, which is when Islam began to spread (about 600-700 AD as compared to 0 AD which is when Christianity began to srpead), you will find that early Christianity was in no way spread through violence or coercion. In fact people died and were fed to beasts in the Coliseum as punishment for their Christian faith by the Roman Empire. Remember Nero? So no, we're talking about distinct differences here. We're talkinga bout Islam, since its inception, gaining converts through violence and Christianity gaining converts through peaceful resistance amidst persecution (remember Dr. Martin Luther King, Jr.?)
So, one cannot argue and say Islam is corrupt and became violent and now it's what it is. We can say that about Christianity, that at one point it became violent and at one point, it converted people through force. However, nowhere in the New Testament from cover to cover will you ever see the use of violence being encouraged as a means of gaining converts.
In contrast, the Koran in its original form does advocate the use of violence for gaining converts, to the point of death, and this is true (and practiced) to this day. To such an extent that Islam desires gaining an Islamic state world-wide so that there will be no infidels and Allah will be pleased. They are willing to die for this.
In fact, let's quote the Koran.
"Prophet, make war on the unbelievers and the hypocrites, and deal harshly with them. Hell shall be their home: an evil fate." (Sura 9.73)
"When you meet the unbelievers in jihad, chop off their heads. And when you have brought them low, bind your prisoners rigorously. Then set them free or take ransom from them until the war is ended." (Sura 47.4)
**Note that these people are doing exactly what their scriptures say: chopping off the heads of the infidels.
"Mohammed is Allah's apostle. Those who follow him are ruthless to the unbelievers but merciful to one another." (Sura 48.29)