Adam Clarke on Matthew 8:22

Let the dead bury their dead - It was usual for the Jews to consider a man as dead who had departed from the precepts of the law; and, on this ground, every transgressor was reputed a dead man. Our Lord's saying, being in common use, had nothing difficult in it to a Jew. Natural death is the separation of the body and soul; spiritual death, the separation of God and the soul: men who live in sin are dead to God. Leave the spiritually dead to bury their natural dead. All the common offices of life may be performed by any person; to preach the glad tidings of the kingdom of God is granted but to a few, and to these only by an especial call; these should immediately abandon worldly concerns and employments, and give themselves wholly up to the work of the ministry.