Well, although it was a druidic celebration, the Church Christianised it. It is Hallow (Holy, i.e. Saints') e'en (even = eve). But as the society becomes more pagan, the eve of the great feast of All Saints becomes more wicked. In Australia, it was only ever a strange word and alien to us, but since we've had imported entertainment which advertised it, it's caught on over the years. The same for Italy. It didn't exist. It's real Americanism at its worst.
It seems the day as a secular quasi-holiday was not really in much practice at
the time of the revolt of Martin Luther. There may have been a druidic tradition
already then, but there was no Halloween as we know it. That came later, and
its start was in America, as you say. Luther nailed his 95 rhetorical ramblings on
the Wittenburg church front door in 1517 long after the Church had already
established the Feast of All Saints.
To say "the Church Christianized it" is to follow the false assertions of the
Protestants who look for every way they can find to be critical of the Church,
even if it means promoting lies. Here
is a pretty good reference for what history has
to teach us regarding what has really happened, instead of what heretics say
in their closed circuits of erroneous contumely.
The Church, in fact, has been practicing All Saints' Day for over a thousand years.
And All Souls' Day for 500 years, started by the Dominicans. Curiously, it was
just after that when Luther got his bright idea, and one has to wonder if his
choice of October 31st to vandalize a Church door had anything to do with his
contempt of the Masses being offered for the faithful departed. But nobody was
trick-or-treating yet. So the Church did not "Christianize" Halloween. I know that
you didn't actually say that, but what you said could easily be interpreted to
say that if someone isn't paying close attention.