das A und O
the essential thing; the be-all and end-all
Origin
The phrase refers to the first (Α, alpha) and last (Ω, omega) letters of the Greek alphabet; it was popularised above all by the Bible ('I am the Alpha and the Omega', Revelation 1:8), where God denotes beginning and end.
Building blocks
Words in this expression
Examples
Gute ist das A und O bei einer .
Good preparation is the be-all and end-all of an exam.