“Opportunity may knock only once but temptation leans on the door bell”

Variant: If opportunity doesn’t knock, build a door.

"Conversation with a Stone"
Poems New and Collected (1998), Salt (1962)

5-Oct-2005, DCFC website
Said after 9 games without a win.

Source: The Purpose Driven Life: What on Earth Am I Here for?

The source is actually a 1945 book by Bruce Marshall, The World, The Flesh, and Father Smith, in which he says, "...the young man who rings the bell at the brothel is unconsciously looking for God."
Misattributed

Solness, Act I
The Master Builder (1892)

Speech in the House of Commons (24 November 1976) http://www.margaretthatcher.org/document/103146
Leader of the Opposition
Context: The word “equality” is often used, but, wisely, rarely defined. The moment one tries to define it, one gets into great difficulty. For example, it cannot mean equality of incomes or earnings; otherwise, we would not need more than one union. Indeed, we would not need one union. If we are to have opportunity, we cannot have equality, because the two are opposite. We may have equality of opportunity, but if the only opportunity is to be equal, it is not opportunity.

Page 226.
The Thought Gang (1994)