Fyodor Dostoevsky's "The Brothers Karamazov".
When answering to Ivan's challenge, first I must put myself in Alyosha's position. Since Alyosha is a devout Christian, he should understand that every Christian should act and think according to the holy bible. There are three reasons that can help Alyosha decide whether or not to create this fabric of human destiny with the ultimate goal to make men happy in the end, even though it means to torture to death one tiny creature: following the path of Christianity, consideration of free will, and the ultimatum of faith.
By following the path of Christianity, an obvious example comes to the mind. Alyosha, as a Christian, should think of the story of Jesus Christ, God's own son. God send his own son Jesus to Earth in a human form to save the whole world but it was essential to torture him and crucified him till death at the end. This scenario is very similar to the one that Ivan gave to Alyosha. God did it with his own son for the sake of the rest of mankind. What looks like immoral and wrong in common sense, does not necessary mean it is wrong in the bible. The cause of this sacrifice serves a great purpose. Our goal is not the life we valued now but it's the life after death we spend with god. As a Christian we believe that death only occurs to our body, our souls, though, lives on. What we experience in this life is nothing compared to what paths lay ahead for our souls. All the pain and sufferings in this life therefore means nothing if you have true faith in what God will provide for us after death. If Alyosha is giving the opportunity that Ivan's challenge is providing, Alyosha should be accepting because he is a believer in God and God did the same. The innocent creature is aiding all of mankind to have a better world now, the one we shouldn't put too much value on to, sacrificing himself by going through pain and suffering, but ultimately serving God, leading to a better life after death.