Again we see the Lord sending an angel to someone whose lifestyle would not seem to deserve the visit (see commentary for Mosiah 27:11). This visitation comes not by Amulek's faith but by that of Alma who had labored in the spirit, wrestling with God in mighty prayer (Alma 8:10). Just as the faith of Alma the elder had opened the heavens to send an angel to his wayward son, Alma the younger's faith was now operative in Amulek's visitation. Amulek was to be the answer to Alma's fervent prayer.
Angels appear unto men by the exercise of faith.
'Behold I say unto you…neither have angels ceased to minister unto the children of men.
For behold, they are subject unto him, to minister according to the word of his command, showing themselves unto them of strong faith and a firm mind in every form of godliness.
…for it is by faith that miracles are wrought; and it is by faith that angels appear and minister unto men; wherefore, if these things have cease wo be unto the children of men, for it is because of unbelief, and all is vain' (Moroni 7:30-37).