I work as a missionary campus pastor. While I get questions from all over the spectrum, most of them aren't as esoteric as this one. Leave it to a specialist in ancient history to ask a question! Here we go, though.
Marcion lived from 85-160. He maintained that Jesus was the real and good God, as compared to the creator God of the Old Testament. Marcion identified himself as a follower of the apostle Paul's teaching. He made the first known list of Christian scriptures, but he reflected his interpretation of what should be there. This was very different from what Christians as a whole recognized. Marcion's list included only ten letters of Paul and Marcion's version of a Gospel, which was reportedly similar in some ways to Luke's Gospel.
Marcion saw a high degree of discontinuity between the Old Testament and the New, clearly viewing Jesus as the good God, unlike the one who created all things. That one he called the Demiurge, and viewed him as petty and vengeful. Jesus, according to Marcion, was only somewhat incarnate, as Marcion took a "docetic" view, saying that Jesus only seemed to have a body.
Marcion was roundly criticized by more orthodox Christians, and was excommunicated in 144.
Was Marcion a Gnostic? His concept of God is similar in some ways, but not entirely. He really doesn't fit well into any of the categories typical of Gnosticism.
Marcion definitively departed from historic Christianity through his denial of the Trinity and his docetic view of Jesus. While he affirmed the truth of the Old Testament, he saw it as something which did not apply to Christians. His view of discontinuity cannot be reconciled with Paul, much less with the canonical depictions of Jesus, who affirmed the validity of the Old Testament. Marcion, therefore, is to be rejected.
My questioner asked why Marcion came to the conclusions he did. One one level I'm sorry I can't flip his head open and take a look in there to find out. On another level, I'm very glad that I can't do that.
Most people who fall into docetism, saying that Jesus only appeared to be incarnate but wasn't really, are trying to defend the glory of his being as God. This is quite likely why Marcion would have taken that point of view. The eternal God who walks around as a human, really as one of us, seems not only impossible, but unthinkable. Yet the New Testament confesses that Jesus was able to do exactly that. As a bodily human, Jesus, God the Son, was able to suffer just as we do, and triumph over little things like sickness and death.
The extreme level of discontinuity between the Old Testament and the New has always puzzled me. After all, Jesus affirmed the unity between himself and the Father. And the Old Testament frequently depicts God as the one whom we praise because of his excellent works and his character full of mercy.
Marcion went astray. He did not repent of his views when confronted, and was removed from communion, becoming someone who had no place for partaking of the body and blood of Christ for his forgiveness. It's a sad chapter in Christian history whenever that happens. We are called to take heed to God, as He is described in the Scriptures. The Lord your God is one.
I've heard this question, often phrased simply as a statement of truth, many times. I'm glad I can have this conversation once in a while. For the direction of my answer, I am thankful to my friend Dr. Adam Francisco, of Concordia University Chicago, who helped me sharpen my thinking.
We really need to get the person asking the question to do a little explaining. "What do you mean by 'just the same'"?
The usual answer has something to do with there being just one deity and being a religion with a strong ethical code.
When presented with that answer, there are a few places we don't want to go immediately. Those are places which will instantly provoke anger and end all interaction. But there are some directions we can take the conversation which can make the exploration work.
Christianity has a moral code. We want to recognize and embrace that. However, the moral code isn't at the heart of Christianity. What is at the heart of Christianity? Jesus the Christ, the second person of the Trinity, who has died to pardon our sins.
We probably don't want to ask a big huge question in return, but if we did want to do so, it would be something like this. "Are you saying that Islam and Judaism confess one God in three persons, Father, Son, and Holy Spirit, and that they say God in the person of the Son, Jesus, gave himself to take on a human nature, to suffer, and to die for the sins of the world, then rose again from the dead to show you that you will also rise to eternal life if you believe in Him?"
Like I said, that's a loaded question. It's too big. But it describes Christianity and can never describe Islam or Judaism. If you believe what that question set out, I have reason to think you're a Christian.
The big question also shows a distinction from all the non-Christian religions. It is set apart because of the nature and actions of God.
For about 2,000 years, Christians have confessed this and many have believed in Jesus as their life and salvation.
I hope my non-Christian questioners will believe in him as well. He is the only one who is victorious over death itself on their behalf. His name is the only name which can be believed on to give eternal life. So we embrace the distinction, and gently call people to recognize who the Christ is and why his work is central to all of life.