And though they say, The LORD liveth; surely they swear falsely.
Though they say, The Lord liveth - (Titus 1:16, "They profess that they know God, but in works they deny Him").
Swear falsely - not a judicial oath; but their profession of the worship of Yahweh is insincere (Jeremiah 5:7; Jeremiah 4:2). The reformation under Josiah was merely superficial in the case of the majority.