It's partly the surprise - I didn't know that they still come to the door - but it's mainly the anger. The baby has just gone to sleep and now someone has started to poke at our doorbell with unbounded enthusiasm.
The bell has transmogrified from a standard battery-operated domestic door buzzer into the loudest piece of electronic equipment in the world, and the baby is already making the ominous gurgling-cum-growling sound that is invariably a precursor to 30 minutes of weeping and wailing.
Whispering dark obscenities, I snatch open the front door to find two plump black women in their later years, apparently dressed for a funeral. They smile the broad and sparkling smiles of the genuinely happy.
The older, larger lady holds a Bible. "Have you heard," asks her younger, smaller but in no way less striking colleague, “the good news?"
At first I am lost for words and simply stand and stare at their hats before noting that, even though it is nearly June, they are wearing gloves.
“Good news?" I manage.
“Yes," the woman continues. "Jesus Christ has come to save you."
Of course - they are Christians. Not the standard British Christians who serve as comedy extras in our national debates - the plum-vowelled men in purple cassocks seeking outreach with jihadis or the angry Presbyterian couples demanding the right to evict sodomites from their Highland tearooms - but West Indian Christians.
They are women in their sixties, with ankles that have started the long slip south and bottoms that seem set to join them, yet they still contrive to walk the suburban streets of this country that has been so careless in its attention to them, offering the greatest gift they have - love.
Most, if not all, doors will be shut on them, given white suburbia's deep distrust of black people or bafflement at such unadulterated happiness. But on the women will walk, spreading the good news as laid out by a minor Palestinian preacher 2,000 years ago.
Which is fine, and in many ways admirable, but the fact remains: they are making noise. "The baby's asleep," I point out.
“Oh," says the older lady, speaking for the first time.
“You are blessed. I am so happy for you." And looking in her eyes, I see nothing but warmth of feeling. But it is too late, the rejection is already coming out of my mouth. "Sorry," I snap,
“I'm not interested." And I shut the door sharply on the Christians.
Through the frosted glass I see the two women do not move immediately, but give me a moment's grace in case goodwill or good manners induces me to change my mind. A pang of regret passes through me and from upstairs I hear the first demented cries of the child.