In his festive reflections, the archbishop said the "dirty, makeshift, less-than-ideal" birthplace of Jesus may have been "embarrassing or shameful" for Mary and Joseph.
"The Christmas story is so familiar, it’s easy to forget it turns the whole world upside down," he tweeted.
He said: "This is God taking the risk not just of being a vulnerable human baby – but being born in the last place you’d expect to find him.
"Picture the scene of Jesus’ birth – dirty, makeshift, less-than-ideal. Perhaps embarrassing or shameful for Mary and Joseph. But God just wanted to be with us.
"This is God saying there are no people I will refuse to love. This is God saying there are no places I won’t bless. This is God saying whoever is outcast, you will find me among them.
"This is God saying things don’t have to be neat and tidy for me to love them.
"God meets us wherever we are, however messy. If you’re embarrassed or ashamed, God is neither.
"So whatever your Christmas looks like – full of joy or sadness, surrounded by people or alone – my prayer is that you know this: God is with you. That’s what 'Immanuel' means – 'God with us'."
"That’s the miracle. Wherever you are, I wish you a Happy Christmas."
Mr Welby, leader of the Church of England, recently reflected on the number of rough sleepers and people using food banks, while sharing his Christmas message with the Big Issue.
The archbishop will deliver his Christmas Day sermon at Canterbury Cathedral on Wednesday.