For many parishes the New Evangelisation will start here - leaving the door of the church not simply unlocked, but wide open. This is especially important when Mass is being celebrated. Many churches do leave their doors unlocked, but for the newcomer or passer by the door may as well be locked. The church door needs to visibly open. An open door is both a sign of welcome and a sign of life. Some of the parishes where I have been Parish Priest liked to keep the church doors closed during the Mass; this is a sign that a congregation has gone into 'private members club' mode and that the focus of the parish is inward looking, or has become, simply, neo-pelagian. I had to work hard to convince my parishioners that an open door was a better sign to give. In one parish, the church was on a main road which clogged up with peak-hour traffic twice a day. I was so glad that when the doors were eventually left open, the slow stream of drivers crawling along could look straight in through to the altar and see the early morning Mass happening. Moreover, a slow realisation grew in the district that this was one church that was actually used each day - becuase, unlike the others nearby, people could see that this one was opened up everyday and that daily Mass was celebrated there.
Incidentally, the churches where I was Parish Priest all had a second set of glass doors within, which kept the church warm in the colder months.