It is a very religious term, but it’s not, in fact, a Christian term, which is one of the oddities of so many people who are self-professed Christians using the term.
St. Augustine, the great Christian theologian, fought battles with other religious figures in his time, like the Manicheans, who stressed evil so much that nothing was left to the proposition that God is good. The idea that God is good is a fundamental proposition of Christian theology.
There’s apparently a reluctance on the part of Christians to use the word “sin” in the public square—they’re much more likely to use the word “evil.” Using the word “sin” might remind Christians that this is something that can be overcome with God’s help, and there’s grace even for the biggest sinners if they find Jesus in their hearts. You can’t be irredeemably evil from a Christian theological perspective, because then there would be no salvation, and no role for Jesus. “Evil” is much more of a secular word than a religious word. “Sin” would be the religious word.
–Alan Wolfe, interviwed by Emma Green.