Says who?
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Incorporated ideas to make those of the other faith understand, yeah. However, you seem to have an idea of the Pope of days of old sitting upon his throne in the Vatican and saying, "Hey, we can't get those pagans in Mexico to worship our God, what should we do? Any suggestions guys?" "I think we should turn one of their important deities into a saint. What about that woman deity?" Of course not.
I still believe you're wrong.
What is more likely to have happened is one of the Pagans converted, and saw similarities between Tonantzin and Mary, and when he began preaching to his friends, he pointed out the similarities, and when someone else converted because of him, they added a bit more of Tonantzin, using pagan imagery and ideas to communicate the message across. "You see her over there? She's not a goddess, she's a servant of the one God! You see, we've been worshipping the creation, not the creator" and stuff. As time goes by, more and more is used, and one ends up with a mainstream bunch of people who are still revering the same deity, but who plays less of a role. Eventually, all the new converts seem to hold a similar belief, but they're still calling themselves Catholic, so the church says "Sure, I guess, if you can see the similarities between this goddess and Mary, then sure, she's Mary. Anything if it helps you in your following of God." or something like that. Deliberate? Not really, just a small sacrifice for them to make to bring people officially into Catholicism. It happens in many religions.