What does the American idiomatic expression "2 x 4" exactly mean? I've read a very interesting book by Father Donald H. Calloway, No turning back (an autobiography and a conversion story), and chapter 10 is entitled "The divine 2 x 4". Could that mean the divine surprise, the divine intervention?
2x4 (two by four) is a standard size piece of wood (in nominal inches).
Being hit by a 2x4 is a common idiom for sudden understanding or for surprise.
I imagine in this case Father Calloway has had a sudden understanding of something to do with religion, hence him describing the 2x4 as divine (from a God).
Edited to add some details from the comments below
'2x4' is the nominal cross-sectional size by some number of feet in length.