One significant difference is that Jesus taught non-violence and Christianity initially spread through peaceful means, through the truth of what was taught, whereas Islam initially spread through violence, initiated by its founder.Yes, just like the crusades, oh and the inquisition, oh and the Nazis. People have committed atrocities while claiming to be Christian too. No human group is exempt from evil and the same criticism you're giving can be thrown right back at us.
Firstly, context is very important:"When he has brought out all his own, he goes before them, and the sheep follow him, for they know his voice." -John 10:4
"And I have other sheep that are not of this fold. I must bring them also, and they will listen to my voice. So there will be one flock, one shepherd." -John 10:16
I take this together with my favourite parable, the parable of the workers in the vineyard. The reason it's my favourite is because it's an example of Jesus turning everything you might think on its head.
"Take what belongs to you and go. I choose to give to this last worker as I give to you.15 Am I not allowed to do what I choose with what belongs to me? Or do you begrudge my generosity?’16 So the last will be first, and the first last.” -Matthew 20:14-16
The key principle is that not everyone gets what they deserve. None of us do. I don't make distinctions based on culture or geography. We are all human and we are all one in Christ. We are also all sinners equally. I have lived among Muslims i the Middle East. My father works with Hindus and Buddhists. There are many good, moral, ethical people among them. Most importantly though, anyone can be led by the Holy Spirit, and if you see the fruit of someone's life, you can tell if the Holy Spirit is at work on them. Remember from back in John 10. The sheep know his voice and they follow him. They do not need to be of the same culture or same system of beliefs in order to be under grace and in order for the Holy Spirit to work through them. We are all one under God and Christ. This doesn't diminish the message of the cross. The grace is available to those people too, and it is manifest through the work of the Holy Spirit.
John 10:1-6, 1 "Truly, truly, I say to you, he who does not enter the sheepfold by the door but climbs in by another way, that man is a thief and a robber. 2 But he who enters by the door is the shepherd of the sheep. 3 To him the gatekeeper opens. The sheep hear his voice, and he calls his own sheep by name and leads them out. 4 When he has brought out all his own, he goes before them, and the sheep follow him, for they know his voice. 5 A stranger they will not follow, but they will flee from him, for they do not know the voice of strangers." 6 This figure of speech Jesus used with them, but they did not understand what he was saying to them. 7 So Jesus again said to them,"Truly, truly, I say to you, I am the door of the sheep. 8 All who came before me are thieves and robbers, but the sheep did not listen to them. 9 I am the door. If anyone enters by me, he will be saved and will go in and out and find pasture. 10 The thief comes only to steal and kill and destroy. I came that they may have life and have it abundantly. 11 I am the good shepherd. The good shepherd lays down his life for the sheep. 12 He who is a hired hand and not a shepherd, who does not own the sheep, sees the wolf coming and leaves the sheep and flees, and the wolf snatches them and scatters them. 13 He flees because he is a hired hand and cares nothing for the sheep. 14 I am the good shepherd. I know my own and my own know me, 15 just as the Father knows me and I know the Father; and I lay down my life for the sheep. 16 And I have other sheep that are not of this fold. I must bring them also, and they will listen to my voice. So there will be one flock, one shepherd." (ESV)
What we see then even here, is that Jesus is "the door," that no one will be saved if they do not enter by him. This agrees with what he says later in John 14:6 "Jesus said to him, "I am the way, and the truth, and the life. No one comes to the Father except through me" (ESV). So just how does one enter through that door? We look at some other things that are said. Consider Peter and Paul for example:
Acts 2:37-38, 37 Now when they heard this they were cut to the heart, and said to Peter and the rest of the apostles, "Brothers, what shall we do?" 38 And Peter said to them, "Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. (ESV)
Romans 10:9-13, 9 because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. 10 For with the heart one believes and is justified, and with the mouth one confesses and is saved. 11 For the Scripture says, "Everyone who believes in him will not be put to shame." 12 For there is no distinction between Jew and Greek; for the same Lord is Lord of all, bestowing his riches on all who call on him. 13 For "everyone who calls on the name of the Lord will be saved." (ESV)
So we can see that in the least, one must repent, "confess...Jesus is Lord and believe in your heart that God raised him from the dead," in order to be saved. And even more than that, verse 13 is a quote from Joel 2:32a: "And it shall come to pass that everyone who calls on the name of the LORD shall be saved." Note that in Joel, "LORD" is Yahweh. It is likely then that Paul is saying in verse 9 that one must confess Jesus is God. Being "good, moral and ethical" does not mean that one is being led by the Holy Spirit or is saved.
All that to say I don't know how one can belong to a belief system which rejects the deity of Jesus or his death and physical resurrection, or both, and yet that person be considered to be being led by the Holy Spirit. Of course one could become saved within one of those belief systems and be trying to find a way out without losing their life, but in no way can one hold both the teachings of Christianity and the teachings of Islam, Hinduism, or Buddhism to be true.
Secondly, we are all human and we all have sinned, but in no way are we all one in Christ. The NT makes it clear that only those who are believers are one in Christ. Maybe that's what you were saying but it comes across as though all humanity is one in Christ.