Zhang Kai, who was released in March this year, has expressed his gratitude in the note for those who supported him during six months he spent behind bars.
In the note (pictured below), penned last Thursday, the prominent human rights lawyer wrote: "Thanks to those comforted and cared for my parents when I was in trouble, last year.
"...I thank the friends, lawyers and the strangers who appealed for me and supported me.
"Thanks to all the Christians at home and overseas, who are praying for me all of the time."
Zhang Kai was detained - along with his two legal assistants - in August 2015 after providing legal representation for around 100 churches targeting in a campaign by officials to demolish crosses on church buildings in Zhejiang province.
He was accused of 'endangering national security' and 'gathering crowds to disturb the social order', allegations China Aid claims he has later pressured into confessing during a television appearance.
China Aid, an organisation which supports persecuted Christian and promotes religious freedom in China, claims Zhang was held "incommunicado in an unofficial 'black jail'".
In his letter, which coincided with the US holiday of Thanksgiving, Zhang Kai said: "The history of Thanksgiving is a history of escaping from oppression, seeking freedom and thanking God.
"On this special day, even though I often feel desperate and devastated, I deeply know in my heart that God always encourages and supports me with His irresistible love, and my gratitude is always beyond words.
"Thanks to the scholars, lawyers, citizens, [etc.], who are devoted to bringing rule of law, human rights and freedom to China. It is you who fill the ridiculous world with love and warmth."