Saeed Abedini says he saw Christians "with very strong faith" turn to Islam in prison as pressures mounted, and has urged his followers to "keep going, no matter what the circumstances".
Abedini, a Christian convert, was freed from Iranian prison in January, where he had been serving an eight-year sentence on charges of "undermining national security".
In a Facebook post he said the difficulties of prison took their toll on some of his fellow inmates.
Many "religious and political prisoners who started out with very strong faith, thoughts and actions while going through so many changes in their country and world" were worn down by jail, Abedini said.
"I saw some Christian[s] who were put in prison because they would not deny their faith and they kept doing their ministry. But unfortunately, I saw some of these heroes of the faith weaken through the difficulties and hardships of prison," he said. "I heard one of the Christians prayed Namaz Islamic prayer to make his situation easier."
Abedini added: "I saw some political prisoners who started obeying the Intelligence Police of Iran to get free or make their situation easier in prison. We do not know how strong our faith really is until it is tested."
He urged his Facebook followers to hold fast to their faith, whatever difficulties they faced.
"Jesus has a reason for telling us to finish what we have started with Him and keep going forward in our Biblical values. Our reward is waiting at the end of our journey," he said.
"I started a ministry that had thousands of people but so many are not serving the Lord anymore because they did not calculate the price to walk this road to the end. Sometimes we want to blame others for causing us to fall away. We have to take responsibility for our walk with the Lord.
"We should not turn back from God's calling on our life. Keep going, no matter what the circumstances. He is waiting at the end of this life's road to take your hand and say 'Well done, good and faithful servant'. To Him be the Glory."
Iranian-born but with duel US-Iranian citizenship, Abedini was arrested in July 2012. He was found guilty the following January of having "undermined the Iranian government by creating a network of Christian house churches and ... attempting to sway Iranian youth away from Islam". Originally held in the notorious Evin prison, he was moved to Rajai Shahr prison later in 2013.
He was freed as part of a prisoner-swap on January 16 this year following a high-profile campaign for his release.
