Explanation:

The eternality of the soul is only relative with respect to the body that is destroyed after some time (Hanyamaane…—Gita). This relative eternality of the soul is taken as a simile for the absolute eternality of God, because there is no absolutely eternal item in the world that can be compared to God. Only the best available item among the relatively eternal items can be selected for comparison. In the beginning of the Gita this subject of eternality of the soul with respect to the body is clearly explained, which gives the reason for the selection of the soul as a simile for God.