The name of all three murdered women are allusions to the figure of Jesus Christ: Gotteskind (Marie Gotteskind, played by Marielle Heller, meaning "Child of God" as pointed out by TJ (Astro); Andersen (Leila Andersen, played by Laura Birn), meaning "Son of Man"; and Kristo (Carrie Kristo, played by Razane Jammal), from which the title "Christ" derives.
Originally, this was supposed to be Joe Carnahan's directorial follow up to his indie hit Narc (2002). At that time, Harrison Ford was attached to star.
(at around 6 mins) The year jumps from 1991 to 1999. As soon as the year changes to 1999 Scudder is sitting in the diner reading the Daily News. However, the back page of the newspaper is a story about a Seton Hall Pirates basketball game from 1991 (the front page looks to be era correct as there is a story about Y2K).
When Kenney selects the cleaver as his weapon of vengeance from the basement sink, it is still in the sink later, even though he is lying halfway down the basement steps.
At the beginning of the movie Scudder is firing an M1911 pattern pistol at the fleeing robber. There is no similar weapon on the approved carry list for a regular (carrying a badge and not undercover) officer of the NYPD.