"nainam chindanti shastrani
nainam dahati pavakah
na chainam kledayanty apo
na sosayati marutah" (Bhagwat Gita: Chapter Two verse 23)
"Sri Krishna said: The soul can never be cut to pieces by any weapon, nor burned by fire, nor moistened by water, nor withered by the wind."
If a person becomes firm in that belief only that soul inside his body is immortal, then too he attains immortality as then that person goes beyond the visible nature which has formed this visible body of ours.
Now what is immortality? This soul of ours wears this body made of nature elements. After karmic cycle, depending on the Karmas of our previous life, soul drops this body at death & takes on another body based on the Karmas of our present life. Again we do Karmas in our next birth & that becomes the basis of our further next birth for this soul. So this cycle of wearing different bodies goes on for this soul. As we all feel pleasure & pain in one's life, so can't it be possible that we can avoid this pain that we feel in our body.
As per 'Bhagwat Gita-Chapter Eighteen Verse - 55', when we go to a perpetual divine life, which is beyond this nature, which forms this body then that life is beyond all this pain & is a blissful life. To attain that we have to establish our self in our soul. As God is the Supreme Soul from whom all these souls have emanated, so worshipping him constantly, one shall be established in the soul & shall attain a blissful life.
"bhaktya mam abhijanati
yavan yas chasmi tattvatah
tato mam tattvato jnatva
visate tad-anantaram" (Bhagwat Gita: Chapter Eighteen verse 55)
"Sri Krishna said: O Arjuna, One can understand Me as I am, as the Supreme Personality of Godhead, only by devotional service. And when one is in full consciousness of Me by such devotion, he can enter into the kingdom of God."
Jai Sri Krishna