Charles H. Spurgeon on Psalms 116:10

The heart and tongue should go together. The tongue should always be the heart's interpreter, and the heart should always be the tongue's suggester; what is spoken with the tongue should be first stamped upon the heart and wrought off from it. Thus it should be in all our communications and exhortations, especially when we speak or exhort about the things of God, and dispense the mysteries of heaven. David spake froth his heart when he spake from his faith. I believed, therefore have I spoken. Believing is an act of the heart, "with the heart man believeth"; so that to say, "I believed, therefore have I spoken, "is as if he had said, I would never have spoken these things, if my heart had not been clear and upright in them. The apostle takes up that very protestation from David (2Co 4:13): "According as it is written, I believed, and therefore have I spoken; we also believe, and therefore speak"; that is, we move others to believe nothing but what we believe, and are fully assured of ourselves. Joseph Caryl.