The Doorpost Cross means life over death. This page explains why.
Conveniently, 'T' was the first letter of the name of the god Tammuz; the dying and rising god, a deity of life-death-rebirth. During baptism ceremonies, this cross was marked on the foreheads by the Pagan priest.
More recently, in the Middle Ages, marking a cross on the doorpost or on the door itself, was a warning to people not to enter. It signified that somebody in the house had died of the pestilence. (See Plague Cross.)
Irish folklore advises that a cross on the door will protect a house from fire (see St. Brighid's Cross) but since most house fires start within the home (electrical faults or cigarette ash a cross on the door is unlikely to be very effective.
Today, hanging a cross on your doorpost is akin to wearing a Lapel pin cross. Before visitors even knock on your door, they assume much about your religious beliefs. (And this should be borne in mind if you think cross jewellery is juat a superstitious charm like a lucky horseshoe.)
Also, don't assume that a cross on the doorpost will in any way Bless This House; and who needs a house blessed anyway? Blessed occupants should be the primary concern.
Blessing, as you probably know already, comes not from a cross on a doorpost but from the love of God. Protection of life, and here we mean the more important spiritual life rather than just the physical life, also comes from the love of God.
See the real meaning of the Cross.
See also Easter
Smoking is hazardous to your health in more ways than the obvious one.
In 1987, a US government study found that cigarettes could be made in a way where starting accidental fires was less likely. And yet they are currently and increasingly the leading cause of fatal fires, resulting in one of every four fire deaths. In 1999, fires related to cigarettes increased by 19 percent over the previous year, according to the National Fire Protection Association (NFPA).
We've no idea whether any of these people had a cross on their doorpost to prevent fire, but even if they did, quitting smoking would obviously have been better.