Arcana Coelestia #2558
2558. 'She became my wife' means that spiritual truth was joined to the celestial, with rationality serving as the means. This is clear from the representation of 'Sarah' when Abraham's 'wife' as spiritual truth joined to celestial good, dealt with in 2507, and from the representation of the same character when 'a sister' as rational truth, dealt with in 2508. Consequently the statement that from being 'a sister' she became 'a wife' means that spiritual truth was joined to the celestial, with rationality serving as the means. What is implied in all this becomes clear from what has been stated immediately above in 2557.
Mercy
In regular language, "mercy" means being caring and compassionate toward people in poor states. That's a position we are all in relative to the Lord, all the time. Without Him we would be unable to choose what is good; without Him we would be unable to formulate a reasonable thought. Without Him, in fact, we would instantly cease to exist; we have life only because He constantly gives us life. So we are, quite literally, at His mercy. Fortunately, the Lord is caring and compassionate to a degree we cannot fathom. He is the source of all caring and all compassion, and of love itself. His mercy toward us never lessens, never abates, never ends; His whole purpose is to bring each of us, individually, to heaven. The meaning of "mercy" in the Bible is closely tied to this idea: it represents love in a general sense, and the desire for good that comes from love. It can also represent the desire for good and the ideas that describe it when those thoughts and desires are inspired by love of the Lord.