Final answer:
The lines imply that human beings are made of dust and will eventually return to dust after death, but the soul is separate from the body and does not follow the same fate. This suggests a belief in the immortality of the soul.
Step-by-step explanation:
The lines 'Dust thou art, to dust returnest. Was not spoken of the soul' from 'A Psalm of Life' by Henry Wadsworth Longfellow imply that human beings are made of dust and will eventually return to dust after death. However, the soul is believed to be separate from the physical body and does not follow the same fate as the body. This suggests a belief in the immortality of the soul, which continues to exist even after death.