A mugger brutally punched a 68-year-old parishioner down the steps of her Queens church early Sunday – then snatched her purse and rode off in her car as she lied on the ground in pain, according to authorities and video of the disturbing crime. The victim was approaching the entrance of the St. Demetrios Greek Orthodox Church on 152nd Street in Briarwood around 8:20 a.m. when the much-younger attacker struck, according to cops and the church’s pastor, Father Konstantinos Kalogridis. Video of the vicious attack, which The Post obtained from the church Monday, shows the suspect stalking the woman as...