"Get married. Have a quiver full of kids if you can."

So said unsuccessful presidential candidate and father-of-five Mitt Romney in a recent speech to graduates. It was a conscious echo of Psalm 127.

The psalm - where children are compared to arrows for war - is the inspiration for the Quiverfull movement.

"Like arrows in the hands of a warrior are sons born in one's youth. Blessed is the man whose quiver is full of them. They shall not be put to shame when they contend with their enemies in the gate."

Christians in the movement believe in giving up all forms of contraception and accepting as many children as God gives, both as a sign of obedience to God and in a bid to ensure the future of the faith.

In the US, Quiverfull families frequently reach up to a dozen children with the numbers of adherents in the tens of thousands. But now the movement is gaining popularity in other countries.