Albert Barnes Commentary Jeremiah 3:6

"Moreover Jehovah said unto me in the days of Josiah the king, Hast thou seen that which backsliding Israel hath done? she is gone up upon every high mountain and under every green tree, and there hath played the harlot." — Jeremiah 3:6 (ASV)

Backsliding Israel - The original is very strong: Have you seen Apostasy? i.e., Israel: as though Israel were the very personification of the denial of God.

She is gone up - Rather, she goes; it is her habitual practice.