"Heart of heart" is an idiomatic expression that communicates the deepest, most profound thoughts or feelings one holds in their core being. While there isn't a direct synonym for this phrase, there are several other expressions that convey a similar sense of innermost convictions. These include "inmost heart," "innermost self," "core beliefs," "inner convictions," "true nature," and "deep-seated emotions." All of these phrases speak to the same idea of the most profound, profound beliefs and values one holds within themselves. Whether it be through prayer, meditation, or introspection, it's essential to dig deep and connect with the heart of hearts within us all.