Be not afraid. So he spake on other occasions when his acts had caused terror and amazement (comp. Matthew 14:27; Matthew 17:7). With all their joy and love, the women could not help feeling fear at his sudden appearance and at the nearness of this unearthly yet familiar form.
Go, tell my brethren. He here for the first time calls his disciples his brethren, intending thereby to assure them of his love and good will in spite of their cowardly desertion, and to signify that he was in very truth the Man Christ Jesus, their Lord and their Master, whom they had known so long and so well.
He had called them friends before his Passion (John 15:14, John 15:15); now he gives them a tenderer title; he is not ashamed to call them brethren (Hebrews 2:11). That they go into Galilee. The message is the same as that given by the angel (Matthew 28:7).
It was meant to comfort them in the absence of daily intercourse with him. But they were not to set out immediately; some other incidents were first to befall them. And there shall they see me. Galilee was to be the scene of the most important revelation, though the Lord vouchsafed to individuals many proofs of his risen life before the promised great announcement.
Why St. Matthew mentions none of these we may form conjectures, but we cannot determine (see on verse 16).