Why would Cortes replace the idols with pictures of Our Lady and the saints?

Primary Source Material:
Three halls are in this grand temple, which contain the principal idols; these are of wonderful extent and height, and admirable workmanship, adorned with figures sculptured in stone and wood; leading from the halls are chapels with very small doors. In these chapels are the images of idols, although, as I have before said, many of them are also found on the outside. In the place of these I put images of Our Lady and the Saints, which excited not a little feeling in Moctezuma and the inhabitants.

He declared that if my proceedings were known throughout the country, the people would rise against me; for they believed that their idols bestowed on them all temporal good, and if they permitted them to be ill-treated, they would be angry and without their gifts, and by this means the people would be deprived of the fruits of the earth and perish with famine. I answered, through the interpreters, that they were deceived in expecting any favors from idols, the work of their own hands, formed of unclean things; and that they must learn there was but one God, the universal Lord of all, who had created the heavens and earth, and all things else, and had made them and us; that He was without beginning and immortal, and they were bound to adore and believe Him, and no other creature or thing. I said everything to them I could to divert them from their idolatries and draw them to a knowledge of God our Lord. Moctezuma replied, the others assenting to what he said, That they had already informed me they were not the originally of this country, but that their ancestors had immigrated to it many years ago; and they fully believed that after so long an absence from their native land, they might have fallen into some errors; that I having more recently arrived must know better than themselves what they ought to believe; and that if I would instruct them in these matters, and make them understand the true faith, they would follow my directions, as being for the best.

Afterwards, Moctezuma and many of the principal citizens remained with me until I had removed the idols, purified the chapels, and placed the images in them, manifesting apparent pleasure; and I forbade them sacrificing human beings to their idols as they had been accustomed to do; because, besides being abhorrent in the sight of God, your sacred Majesty had prohibited it by law, and commanded to put to death whoever should take the life of another. Thus, from that time, they refrained from the practice, and during the whole period of my abode in that city, they were never seen to kill or sacrifice a human being.