I will declare thy righteousness - This is evidently spoken ironically. The sense is, ‘you have devoted yourselves to idols, and you have sought the aid of foreigners. I will now announce to you the true nature of the deliverance which they can bring to you.’ This is done in the following verse.
Related Commentaries of Isaiah 57:12
Isaiah 57:12
12 I will declare thy righteousness, and thy works; for they shall not profit thee.