So do you think Islam is very akin to a Christian heresy?
To some. I disagree with them however, though some of the roots of Islam are found in both orthodox and heretical Christianity.
It is its own religion.
Didn't Marcionists reject the OT because they thought that the God in the OT was things like genocidal, incosistent, and wrathful?
That's a major part of it. Do you see now why the Allah=/=God by Divine Character argument isn't acceptable to orthodox Christianity?
Is this the same reason Islam rejects the OT?
I don't know what you've been told, but Islam accepts the Tanakh as Scripture. To surprise you even more, they accept the New Testament also.
Their matter of interpretation is different. Just because they interpret both differently doesn't mean they reject the books; they simply reject our (the orthodox) interpretation of them.
Or does Islam reject the OT because they believe that Ishmael received the promise instead of Isaac, which has nothing to do with Marcionism? Does Islam even reject the OT? I'm only so-so with Islamic theology.
See my above. They accept both the OT and the NT. They would simply say that we've misinterpreted them and injected our own interpretations into them.
Does having a wrong theology imply that you are worshiping a different or a wrong diety?
You're not a Catholic. Do Catholics worship the same God as you? To be Protestant as you are means that you must believe that we Catholics have incorrect theological beliefs and practices. By your implied implication here, that means that we don't worship the same God as you do. And similarly, you would therefore expect us to think Protestants worship a different deity than we Catholics do.
Do you agree or not agree?
What would be the implications on salvation if Muslims have just a wrong theology?
I don't know what you've been told, but whenever has Christianity ever stated that salvation was closed? I would suggest picking up St. Justin the Philosopher and Martyr and read what the Ante-Nicene Fathers really believed about salvation outside their religion.
Grace is a gift and it is a gift found in many ways. The surest ways are the Seven Sacraments, although it is by no means limited to them. Truth in itself is a grace. Doing selfless deeds in cooperation with the Holy Spirit is a means of grace, for it heals the soul and purifies the nature. Even by reading the Bible, reciting a rosary or prayer rope, kissing an icon, or listening to a sermon can be a means of grace.
Muslims hold to a great many truths. Their orthodox (=/=conservative or fundamentalist) beliefs and practices contain many truths and virtues. They do not possess them all, but they possess elements. To deny this is to deny them in Christianity. Muslims do therefore have elements of grace within, but they also have a great many obsticles as well.
So can they be saved? Yes. Will they? I'm not God and nor are you. We are not righteous enough; we are not holy enough. Neither do we have the Divine authority or grace to pass such judgment. We've even been told by Jesus in the Holy Canon not to judge least we judge ourselves in our hypocracy or incorrect judgment. Imagine saying to someone that they're going to hell only to find out that God was merciful to that person in the End and then turns to you, remembering that you condemned that person. God is therefore going to send
you to experience hell if this senario were to unfold!
That's the terrible
danger that lies in playing God. I don't know anyone's eternal fate, least of all my own. What I do know is that my religion contains the wholness of truth and thus the wholeness of grace. Anything that shares in those truths would therefore have elements of the wholness of grace logically. Islam definitely fits that bill. Yet if I cannot know if I'm going to endure to the end, who the heck am I to know whether a person of another religion, Islam besides, will make it? I have my own obsticles, however reduced they are since I am Christian, that I must deal with. Do those obsticles mean that I'm doomed too? Different obsticles, yet obsticles nonetheless.
How much theology needs to be correct?
What is the root of Hinduism? It isn't Abrahamic. What of Animism? Not Abrahamic. How about Shinto? Unless the aliens that helped build the pyramids flew Abraham to Japan, they aren't Abrahamic either!
There are three great Abrahamic religions: Judaism, Christianity, and Islam. One could include the Druze and the Samarians too. These, and perhaps one or two others that escape me at the moment, have historical ties with the faith of Abraham. Their theologies are roughly similar and their practices are roughly the same. Their God is the same, as they all claim the Abrahamic faith and can historically tie themselves to it.