The most straightforward answer is "secular." People who celebrate this day in a religious context generally do not call it Halloween, and the common practices associated with Halloween such as costuming and giving of treats are secular celebrations.
Christian Origins – All Hallows Eve and All Saints DayHowever, Halloween evolved out of a Catholic holiday called All Hallows Eve, which occurs the day before All Saints Day, a general celebration of the saints on November 1.
In turn, All Saints Day originally was celebrated on May 13, and in the Orthodox Church is continues to be celebrated in late spring on the first Sunday after Pentecost, which in turn is seven weeks after Easter. Pope Gregory III is commonly credited with moving it in the 9th century to November 1, although the reasons for the move are debatable.
Ancient Celtic Origins - SamhainIt is often argued, most commonly by neo-pagans and Christians who are against Halloween celebrations, that All Saints Day was moved to November 1 to co-opt a Celtic Irish celebration called Samhain.
Read more: What We Know Do and Don't Know about Samhain
Did the Catholic Church Co-opt Samhain?There is no direct evidence to say they did. Gregory's reasons for moving it from May 13 to November 1 remain mysterious. A twelfth century writer suggested it was because Rome could support larger numbers of pilgrims in November than in May.
There are similarities. Samhain appears to have connection with the dead and may have involved communication with, placating of, or honoring of those who had died. All Saints is a celebration of dead saints, whom Catholics communicate with through prayer and offerings in the hopes of the saints acting as intermediaries between humanity and God.
However, Ireland is a long way from Rome, and Ireland was Christian by the time of Gregory. So the logic of changing a feast day throughout Europe to co-opt a holiday originally celebrated in a small portion of it has some substantial weaknesses.