Hebrews 13:5 quotes God's promise from Deuteronomy: "I will never leave thee, nor forsake thee." The Greek uses a double negative construction — ou mē — which is emphatic beyond ordinary negation. It is closer to "I will absolutely, certainly never leave you." The promise is stated as a fact about God's character, not as something that depends on whether you can feel it. God's presence is not conditioned on your ability to perceive it.
Isaiah 49:15 makes one of the most striking comparisons in the Old Testament: "Can a woman forget her sucking child?" The expected answer is no. But then God says: even if she could, he cannot forget. The image chosen is the most visceral form of maternal attachment — a nursing mother and her infant. God's connection to his people exceeds even that.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.