This is the 7106th most frequent Russian word.
божьей
"Божьей" means "God's" (possessive form).
The word 'божьей' is used here as a genitive form referring to something that belongs to or is associated with God.
Я верю в силу Божьей воли.
I believe in the power of God's will.
In this instance, 'божьей' describes the source of the mercy, attributing it to a divine origin.
Чудо произошло по Божьей милости.
A miracle occurred by God's grace.
Here, 'божьей' indicates whose help was given, specifying that the help is divine in nature.