He made this statement in a radio broadcast to the British people on October 27, 1941, at a time when the outcome of the war was uncertain. The quote was intended to inspire and motivate the British people to persevere through the difficult times. It is believed he made the quote while riding the London Underground, though it's not documented as a fact.
It's important to note that the quote itself was a shortened and modified version of the original Newman quote. The full original quote reads:
"The blood of the martyrs is the seed of the Church: the Church is not only an institution; but it is a living thing, a Body. There is no more to be expected, than that a living Body should have a living energy: that it should defend itself; that it should retaliate when attacked; and that it should be stronger and stronger, and more united and more united, as it grows in the midst of enemies."
However, the shortened version became the most widely remembered quote, and it is often used to symbolize Churchill's spirit of resilience and determination during World War II.