John Gill Commentary Ezra 6:12

"and the God that hath caused his name to dwell there overthrow all kings and peoples that shall put forth their hand to alter [the same], to destroy this house of God which is at Jerusalem. I Darius have made a decree; let it be done with all diligence." — Ezra 6:12 (ASV)

And the God that has caused his name to dwell there
Whose name is not only called upon there, and that called by his name; but who grants his presence, and causes his Shechinah, or divine Majesty, to dwell there, as in Solomon's temple, which Darius had some knowledge of:

destroy all kings and people ;
let them be who they will, high or low:

that shall put to their hand to alter and to destroy this house of God,
which is at Jerusalem ;
this he said to deter from hindering the building of it now, and from attempting to destroy it hereafter:

I Darius have made a decree, let it be done with speed ;
be carried immediately into execution, especially with respect to the disbursement for the building of the temple, and for the sacrifices of it.