Bede : David, however, was the youngest. The Lord is to be understood as the youngest among his brothers for a twofold reason: in which he filled the number eight, that is, those to whom he entrusted the sacraments of the Gospel when they were perfected in the law, both in time and in humility. For when the fullness of time had come, God sent his Son, born of a woman, born under the law, to redeem those who were under the law (Galatians IV). And: The Son of Man came not to be served but to serve, and to give his life as a ransom for many (Matthew XX). For even that grain of the evangelical mustard seed, which he himself sowed in his field, namely in the hearts of believers, is indeed the smallest of all seeds, in the humility of the doctrine of discipline, but foremost in the fervor of spiritual grace. — Commentary on Samuel