Tat Astu-the blessing of "so be it" or more specifically, Tat Asi is " so it is" . Tat tvam Asi is the vedantic mahavakya (great exhortation) , Thou art that, which signifies that everything already is divine!!

Tat Astu-the blessing of "so be it" 
or more specifically, Tat Asi is " so it is" .
Tat tvam Asi is the vedantic mahavakya (great exhortation) , Thou art that, which signifies that everything already is divine!!