2. And thou—rather, "And if (carried on from Jer 4:1) thou shalt swear, 'Jehovah liveth,' in truth, &c.", that is, if thou shalt worship Him (for we swear by the God whom we worship; compare De 6:13; 10:20; Isa 19:18; Am 8:14) in sincerity, &c.
and the nations—Rather, this is apodosis to the "if"; then shall the nations bless themselves in (by) Him" (Isa 65:16). The conversion of the nations will be the consequence of Israel's conversion (Ps 102:13, 15; Ro 11:12, 15).