John did not say practice in his quote that we cannot sin once we are born again. The seed of the holy spirit is powerful...yet christians deny it! Sad.
Educate yourself: "In Greek when the present tense occurs it can be understood in a number of ways, one of which is the habitual present. [...] the habitual present refers to events which occur over and over again repeatedly." - http://www.faithalone.org/magazine/y1990/90march2.html