Adam Clarke on Joel 1:17

So shall ye know - By the judgments I execute on your enemies, and the support I give to yourselves, that I am the all-conquering Jehovah; and that I have again taken up my residence in Jerusalem. All this may refer, ultimately, to the restoration of the Jews to their own land; when holiness to the Lord shall be their motto; and no strange god, or impure people, shall be permitted to enter the city, or even pass through it; they shall have neither civil nor religious connections with any who do not worship the true God in spirit and in truth. This, I think, must refer to Gospel times. It is a promise not ye fulfilled.