It's a common supposition among older style historians that people only converted for power or to gain access to wealth, but more contemporary historians don't necessarily subscribe to that narrative. There's increasing willingness to acknowledge that people--including the ruling elite--typically did in fact believe their own religion. You really have to look at a case by case basis to determine whether conversion was genuine or not. And of course genuine conversion and converting for power or wealth aren't mutually exclusive! One can easily believe that if another people are seemingly wealthier and more powerful than you, clearly their god is better than your god and give more benefits to worshippers.Despite what many monotheists will claim, most pagan peoples adopted monotheism for a firmer grasp on power (this was especially true for the Norse - they didn't see some vague "truth" in Christianity, it was just convenient to access the wealth and alliances of the rest of Europe; it's fairly clear this was the same logic followed by the Anglo-Saxon kingdoms). In India, Hindus already held the power, so there was no real incentive to forsake their native beliefs.
The majority of conversions to monotheism went from the top-down, after all.