Jeremiah 17:5 - Matthew Poole's English Annotations on the Holy Bible

It was the great sin of this people, for which they are often taxed in holy writ, 2 Chronicles 16:7, 2 Chronicles 28:16,20 Isa 30:1,2 31:1,2, when any danger threatened them for their sins, to make leagues with and flee to foreign idolatrous nations to help and succour them, and to repose a confidence in them, and so bolster up themselves in their wicked and sinful courses, promising themselves deliverance from the dangers that threatened them by the power of their confederates and allies. This sin the prophet here reflecteth upon, that while their hearts departed from God, they would yet encourage themselves from the hoped-for help of men. The prophet from God declares that such are and shall be cursed, and mentioned man, under the notion of flesh, to show his frailty and impotency to help against, the mighty power and wrath of God; withal showing us that God alone is the true object of our faith and confidence, as well for the things of this life as those of another life, and that none whose heart departeth from God can with any security look for any help from the creature.