The inner hold a prisoner has on his spiritual self relies on having a hope in the future, and that once a prisoner loses that hope, he is doomed.
He also wrote:
The salvation of man is through love and in love.
If you’ll read the Wikipedia article about his book (linked above) you’ll also see that Frankl learned through his horrifying experiences that the only thing really worth fearing is God.
Such contemplations brought Frankl near to Christ, whether Frankl himself realized it or not.