What is salvation? Salvation is ending up with everlasting life in paradise after this life on earth. The question emerges, what is paradise? Is it for real? And how do you get there?
If you are a Christian, the answer is obvious. Jesus paid the fine on your behalf, the way only the Son of God could. If you accept that gift, you go to paradise. That is salvation. But then the command is that you take that message to every nation on earth.
There are those who don't even believe. They will tell you, there is no God, there is no heaven. And then there are those who seem to practice all these other religions. Either you will have to tell them they have false religions. Walk away from your false religion, come to this true religion. Is that the way to go?
My point is, that is not the way to go. Do you realize how hard it is to establish the idea that there is a God? If there is a God, where is He? Who is He? What is He? Do you realize how hard it is to establish the fact that there is a heaven, there is a hell? For Christians all they have to do to appreciate that is to read the Old Testament, aka the Jewish Bible. Look at how hard it was for God to establish those facts among his chosen people. And they still missed out when he showed up finally, as promised.
On the other hand, you can read that same Old Testament to find evidence of false religions.
My point is that the very scriptures in these other major religions point towards the Gospel.
For example, Muslims are aware there is a God, there is heaven, there is hell, but Prophet Muhammad died not knowing if he was going to heaven or not. If he himself did not know, how could he have taught others how to? Did he go to heaven? I'd be surprised if he did not. But the point is, he did not before he died.
Hindus talk of being born on this earth again and again. They do know there is a God, and that God is a trinity, but they also seem not to know the bridge to heaven.
The most famous Buddhist on the planet, the Dalai Lama, of course, is born on earth again and again. He does not seem to die and end up in paradise.
Prophet Muhammad died not knowing if he was going to heaven. If he did not know, how could he have taught you?
The books that the Muslims and the Hindus read to know there is a God also happen to be books that point them towards the path to salvation.
This is not a my God is better than yours proposition. There is but one God. There is no God but God. This is not a my religion is better than yours proposition. There is no religion in heaven. There is only God.
The human being is finite. God is infinite. God is always going to be more than what you know God to be. And so it makes sense for Christians to get curious about these other scriptures.
The only way to take the Gospel to every nation on earth is by pointing out how these other scriptures point towards the Gospel. The Jewish Bible is not the only one.
When a Christian does that with the Hindu scriptures, for example, that necessarily leads to pointing out hypocrisies in the Hindu society. If a low caste birth is because of sins from previous lifetimes, and if Lord Krishna says in the Geeta that "by way of Wisdom, you will be able to cross the sea of all sin," then that means 300 million Dalits in India can accept Jesus, as per Lord Krishna, and attain salvation for the afterlife, and political and social equality right here on earth, effective immediately. The caste system comes crumbling down.
Just like there are passages in the Jewish Bible that can not be taken out of their historical contexts, that is true also for the Quran. Just like there are detailed laws in the Jewish Bible, there are detailed laws in the Quran. The detailed laws get simplified and superseded by two simple laws, Love God, and love your neighbor.
Some Hindus in India demolished a mosque in 1992 that had been planted at the very place where Lord Rama, the first human incarnation of Lord Vishnu, the Holy Father, was born. That which the Muslims call Allah, the Hindus call Lord Vishnu. That which the Hindus call Bramha, the Christians call Jesus, the Jews and Christians call Wisdom. Bramha and Wisdom mean the same thing.