The parents of Kayla Mueller, who was killed while being held captive by Islamic State (ISIS) earlier this year, have said their faith in God has given them strength to keep going.

Speaking at the first International Congress on Religious Freedom in Madrid this weekend, Carl and Marsha Mueller said their daughter had always wanted to help other people, and had travelled to Turkey to work with refugees after hearing of their plight. She was abducted by ISIS militants after visiting a hospital in the Syrian city of Aleppo on August 3, 2013.

"She was capable of going where the needy were in order to work out how she could help. She believed in freedom and she felt the injustice being suffered by those who were deprived of it; she was tormented by the suffering of others," the Muellers said.

Kayla, 26, wrote in a letter to them: "It should not be a question of 'my people' and 'your people': wherever there is injustice, that is my problem."

The Muellers kept silent about their daughter's capture until ISIS confirmed her death on February 6 this year. The family then released a letter written during captivity in which Mueller said she felt close to God.

"I remember mom always telling me that all in all in the end the only one you really have is God. I have come to a place in experience where, in every sense of the word, I have surrendered myself to our creator b/c literally there was no else....+ by God + by your prayers I have felt tenderly cradled in freefall," she wrote.

"I have been shown in darkness, light + have learned that even in prison, one can be free. I am grateful. I have come to see that there is good in every situation, sometimes we just have to look for it. I pray each day that if nothing else, you have felt a certain closeness + surrender to God as well + have formed a bond of love + support amongst one another..."

She concludes: "Please be patient, give your pain to God. I know you would want me to remain strong. That is exactly what I am doing. Do not fear for me, continue to pray as will I + by God's will we will be together soon."

Now, over two months since they heard of their daughter's death, Mueller's parents say her steadfast faith has been an important part of their journey.

"They [ISIS] not only took our daughter: but they took 18 months of our lives ... Every phone call, every e-mail was a big event for us, always waiting to hear news about Kayla," they said.

"But faith helped us to carry on, the same faith that we could see in Kayla in her letter."

Mueller's mother said that their relationship with God has not changed, despite the trauma. "God is always there, alongside us: always," she said. "God has not angered me at all. As Kayla would say, 'suffering is not something that is normal,'" she said.

"It is difficult to go through the death of a loved one, and when it happens it is normal to ask God why. Perhaps we will never know why, until the day we are reunited with Kayla. Meanwhile, God has given my life the knowledge and strength to carry on," Carl Mueller added.

The Congress, called 'We are all Nazarenes', was held in solidarity with Christians facing persecution all over the world. Guests included two schoolgirls from Chibok, who managed to escape Boko Haram after being captured in Nigeria, Ignatius Joseph III Younan, the Patriarch of Antioch (Syria) and the General Bishop of the Coptic Orthodox Church in the UK, Bishop Angaelos. The family of Asia Bibi, a Christian on death row in Pakistan, were also interviewed, and said that her faith remains strong.

The second Congress on Religious Freedom will be held in the US next year, organised by CitizenGo.