The phrase "unbelieving heart" refers to a person who does not have faith or belief in something. It means that they are skeptical or doubtful about a particular idea, concept, or faith. Full definition
This word is addressed both to the rebellious, unbelieving hearts of the scribes and to the naive, trusting hearts of those who brought their friend on the pallet. (religion-online.org)
What structures do you have in place to ensure, as humanly possible, that an evil, unbelieving heart does not lead you to fall away from the living God (Hebrews 3:12 - 13)? (relevantmagazine.com)
I would argue that our being relevant can be a method by which God may soften and nourish much of the hard, barren soil of unbelieving hearts amidst today's cultural landscape. (relevantmagazine.com)