14 Pascha, as they say who know, and who have explained to us what to read, means “Passover.” When then the Lord's Passion was about to come, the Evangelist, as though he would use this very word, says, “When the hour had come that Jesus should pass over to the Father.” We hear then of Pascha in this verse, “I am alone, until I pass over.” After Pascha I shall no longer be alone, after passing-over I shall no longer be alone. Many shall imitate Me, many shall follow Me.
And if afterward they shall follow, what shall be the case now? “I am alone, until I pass over.” What is it that the Lord says in this Psalm, “I am alone, until I pass over”? What is it that we have expounded? If we have understood it, listen to His own words in the Gospel. “Except a grain of wheat fall into the ground and die, it abides alone; but if it die, it bears much fruit.”...Therefore He was alone before He was put to death....So far was any from dying for the Name, that is, for confessing the Name of Christ, before that Corn of wheat fell into the ground, that even John, who was slain just before Him, being given by a wicked king to a dancing woman, was not put to death because he confessed Christ.
Of course he might have been put to death for this, and that by many. If for another reason he was put to death by one man, how much more might he have been put to death by those very men, who put Christ to death? For John gave testimony to Christ. They who heard Christ, wished to slay Him; the man who gave testimony to Him they slew not....He is not slain by the Jews who gave free testimony to Christ, whom the Jews slew; he is slain by Herod, because he said to him, “It is not lawful for you to have your brother's wife.” For his brother had not died without issue. For the law of truth, for equity, for righteousness' sake, he did die: therefore is he a saint, therefore a martyr; but yet he died not for that Name whereby we are Christians, wherefore, save that the saying might be fulfilled, “I am alone, until I pass over.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)