Whereunto (eis ho). The goal, that is the final salvation (sōtēria). Through our gospel (dia tou euaggeliou hēmōn). God called the Thessalonians through Paul’s preaching as he calls men now through the heralds of the Cross as God chose (cf. 1Th 2:12; 1Th 5:24).
To the obtaining (eis peripoiēsin). Probably correct translation rather than possession. See note on 1Th 5:9, there of salvation, here of glory (the shekinah, glory of Jesus).