Also, goodness knows; heaven knows.
Truly, certainly, definitely, as in God knows I need a winter coat. This expression, which originated about 1300 as God wot, does not necessarily imply that God is all-knowing but merely emphasizes the truth of the statement it accompanies. The variants using goodness and heaven are euphemisms that avoid taking God's name in vain. [Second half of 1500s]
Also, God only knows. Only God knows, that is, neither I nor anyone else knows, as in God knows where I've stored those photos, or God only knows how many people will join the march. [Second half of 1500s]