John Chrysostom — as excerpted in the Catena Aurea on Matthew 5:1-3

“He ascended a mountain, first, that He might fulfil the prophecy of Esaias, Get thee up into a mountain; (Is. 40:9.) secondly, to shew that as well he who teaches, as he who hears the righteousness of God should stand on an high ground of spiritual virtues; for none can abide in the valley and speak from a mountain. If thou stand on the earth, speak of the earth; if thou speak of heaven, stand in heaven. Or, He ascended into the mountain to shew that all who would learn the mysteries of the truth should go up into the Mount of the Church of which the Prophet speaks, The hill of God is a hill of fatness. (Ps. 68:15.)”