After nominating Joshua as his successor, and assigning the keeping of the Law to the priesthood and body of elders, Moses was summoned by the Lord to appear with Joshua in the tabernacle, that Joshua might receive a charge and appointment to his office.
At the same time, God announced to Moses that after his death the people would go astray, and turn to idolatry, and violate the covenant, so that God's anger should be kindled against them, and he would leave them to suffer the consequences of their folly and sin.
In view of this, Moses was directed to write a song and teach it to the people, that it might abide with them as a witness against them, rising up, as songs will do, in the memory of the nation, even after they had apostatized from the path in which the author of the song had led them.