Rashi comments: An altar of stones and an altar of earth He commanded to make, but this He hates, for it was the custom of the Canaanites. And even though it had been beloved by Him in the days of the Patriarchs, now He hates it since these (Canaanites) made of it a custom for idolatry.
The reference to the Patriarchs is to events such as is described in Genesis 21:33:
"And Abraham planted a tamarisk-tree in Beer-sheba, and called there on the name of the LORD, the Everlasting God."
and Genesis 28:18,22:
"And Jacob rose up early in the morning, and took the stone that he had put under his head, and set it up for a pillar, and poured oil upon the top of it...and this stone, which I have set up for a pillar, shall be God's house"
and Genesis 35:14-15:
"And Jacob set up a pillar in the place where He spoke with him, a pillar of stone, and he poured out a drink-offering thereon, and poured oil thereon. And Jacob called the name of the place where God spoke with him, Beth-el."
According to Rashi, God was originally fond of these customs during the time of the Avos but then changed His mind a few hundred years later and decided they were a no-no because the Canaanites adopted the same practices.
Now contrast this with what Rambam says regarding animal sacrifices in Guide for the Perplexed (3:32): The custom which was in those days general among all men, and the general mode of worship in which the Israelites were brought up consisted in sacrificing animals in those temples which contained images, to bow down to those images, and to burn incense before them... It was in accordance with the wisdom and plan of God, as displayed in the whole Creation, that He did not command us to give up and to discontinue all these manners of service; for to obey such a commandment it would have been contrary to the nature of man, who generally cleaves to that to which he is used.
Thus an interesting conclusion can be derived from the opinions of these two Rishonim: if Jews started a custom that was later adopted by Gentiles, it is verboten. God just doesn't like it!
- The Reform were decried for their incorporation of music into their services since this was looked upon as a Christian practice, even though we had long before relied on vocal and instrumental music during Temple services.
- Although the custom of covering one's head is an ancient one, it was once primarily adopted only by the pious and by married men (see Kiddushin 29b); going with an uncovered head - even in synagogue - was customary in the middle ages (see, for example, Shulchan Aruch O.C. 91:3). But, the theory goes, the universal Ashkenazic practice of covering the head became entrenched when Jews became accustomed to seeing Christians church-goers with uncovered heads.
- The Vilna Gaon annulled the long observed custom of erecting trees and greenery in shuls during Shavous because it eventually became a gentile custom.
And, oh yes, the common practice of incorporating gentile tunes into Jewish liturgy - regardless of what certain esteemed rabbinic leaders say about goyishe African music and beats.