giving vengeance to those who don't know God, and to those who don't obey the Gospel of our Lord Jesus,
Dative plural of perfect active participle (eidos). Apparently chiefly Gentiles in mind (Galatians 4:8; Romans 1:28; Ephesians 2:12) though Jews are also guilty of wilful ignorance of God (Romans 2:14). And to them that obey not the gospel of our Lord Jesus...Repetition of the article looks like another class and so Jews (Romans 10:16). Both Jews as instigators and Gentiles as officials (politarchs) were involved in the persecution in Thessalonica (Acts 17:5-9; 2 Thessalonians 1:6). Note the use of "gospel" here as in Mark 1:15 "believe in the gospel" (Robertson 44).