Paraphrasing (shorter version of your question) "why did you choose your religion and not another?"
Irish Catholics might cringe at their son marrying out of their religion (Roman Catholic).
Religion segregates and isolates.
Yet all Christians (for example Mormons) believe in Christ and the same God. (I can imagine other Christians cringing at the mention of Mormons).
Many theists feel blessed by God to have been born to parents with the ONLY true religion (all others are deemed heathen, Pagan, or evil). In their view, God revealed his presence only to a small group of people in the Middle East (not to all of his children). Many feel that the "Old Time Religion" was good enough for grandpa, good enough for father, and good enough for me.
Pioneers often settled in the boonies, surrounded by Native Americans, and often were cut off of their own religion. They had to either settle for the religion endemic to the region, or go without spiritual guidance all together. They might have attended the newly formed Methodist church, or Lutheran church, because, though it was different, they felt that it was close enough to their own religion that they would tolerate it (barely). Over the years, it grew on them, and their kids grew up with it, so they dearly loved their church and their community.
To God, all humans are his children, and all presumably, deeply loved. One wonders why one suffers with cancer and predation (and all of the pain associated with them).
If this is so, then the ancient native Hawaiians, who believed in a volcano God, would also have to be a child of God. Why would God not send a message to all of his children and let his presence be known? Are Hawaiians wrong? They practiced human sacrifice, murdering newborns with blemishes (mark of the devil). But, when one lives on an isolated island, and mates with close relatives (many kings married their sisters), one must be ever vigilant against mutations, and ritual sacrifices of the deformed kept their genome pure. Perhaps, for the Hawaiians, their form of God's law worked, and other forms of God's law would not have? Maybe "thou shalt not kill" didn't apply to the Hawaiians, under all circumstances? I can't blame Hawaiians for worshiping a powerful flame and molten rock spewing God that shakes the earth.
The prevalence of religion around the world is proof that mankind has an innate need for a God (father figure who will come to one's rescue and guide one's actions, and ultimately find room in eternal paradise (perhaps with 72 virgins if you murder his enemies).
Why would a God have to rely on a mere human to murder? Isn't God, the all powerful, the creator of the universe, powerful enough to do his own murdering? Couldn't God, with the wave of his hand, wipe out whole armies? Why would God demand that his own son (Jesus) be horribly tortured to death before he relents to pardon humans for their sins? Wouldn't that make God a cruel God? Who would want to spend eternity right next to a cruel God? If God is evil, and people worship God merely because he is powerful, why not worship Satan instead?
While many theists consider their counterparts Pagan (and often call them that to their faces), their counterparts likewise consider them to be Pagan.
Wouldn't it be a better world if we all treated each other as brothers and sisters (all God's children)?
If God taught rules to other cultures and religions, maybe it is a matter of studying other religions to piece together threads of commonality of all religions? God had initially scrambled languages at the Tower of Babel, and strewn people to far-flung locations. Could it be that God wants us to find peace and brotherhood in our hearts, overcome language barriers, overcome distance barriers, and seek a common religion with all of his children? Maybe Babel was a test, to see if mankind has the capacity to get along on earth, to make sure that mankind could get along in heaven, as well?
