Augustine of Hippo

(Serm. 76.) For in one Apostle, namely Peter, first and chief in the order of Apostles in whom was figured the Church, both kinds were to be signified; that is, the strong, in his walking upon the waters; the weak, in that he doubted, for to each of us our lusts are as a tempest. Dost thou love God? Thou walkest on the sea; the fear of this world is under thy feet. Dost thou love the world? It swallows thee up. But when thy heart is tossed with desire, then that thou mayest overcome thy lust, call upon the divine person of Christ.