Charles Ellicott Commentary Deuteronomy 30:20

"to love Jehovah thy God, to obey his voice, and to cleave unto him; for he is thy life, and the length of thy days; that thou mayest dwell in the land which Jehovah sware unto thy fathers, to Abraham, to Isaac, and to Jacob, to give them." — Deuteronomy 30:20 (ASV)

He is thy life, and the length of thy days. —This is the Old Testament form of a well-known saying in the New Testament, which may yet be fulfilled in Israel, “I am the resurrection and the life. He that believeth in me, though he were dead, yet shall he live; and whosoever liveth and believeth in me shall never die” (John 11:25–26).