In post-exilic times in Persia, God used concealed Jews (exampled by Mordecai and Esther) to ascend to levels of prominence on behalf of their people.
Sin, the real opiate of the people, makes us oblivious to danger, giving us a debased and reprobate mind. It is not static, but leads to destruction.
Just as Mordecai conceals Esther, God conceals His people in secret places under the shadow of His wings, in the sanctuary—the fellowship of the church.
Book Three of the Psalms addresses the compulsion to fast and to mourn. Judah's faithlessness brought about the horrific destruction of Jerusalem on Av 9.
In most biblical contexts, 'spirit' refers to the invisible, internal activating dimension of the mind. Synonyms include heart, mind, and thoughts.