Adam Clarke on Matthew 16:21

From that time forth began Jesus, etc. - Before this time our Lord had only spoken of his death in a vague and obscure manner, see Mat 12:40, because he would not afflict his disciples with this matter sooner than necessity required; but now, as the time of his crucifixion drew nigh, he spoke of his sufferings and death in the most express and clear terms. Three sorts of persons, our Lord intimates, should be the cause of his death and passion: the elders, the chief priests, and the scribes. Pious Quesnel takes occasion to observe from this, that Christ is generally persecuted by these three descriptions of men: rich men, who have their portion in this life; ambitious and covetous ecclesiastics, who seek their portion in this life; and conceited scholars, who set up their wisdom against the wisdom of God, being more intent on criticising words than in providing for the salvation of their souls. The spirit of Christianity always enables a man to bear the ills of life with patience; to receive death with joy; and to expect, by faith, the resurrection of the body, and the life of the world to come.