Pastor Wang Yi wrote in September that he would "categorically" deny any "filth" he was accused of by authorities but he would serve any sentence if jailed.
He said: "Those who lock me up will one day be locked up by angels. Those who interrogate me will finally be questioned and judged by Christ.
"When I think of this, the Lord fills me with a natural compassion and grief toward those who are attempting to and actively imprisoning me.
"Pray that the Lord would use me, that he would grant me patience and wisdom, that I might take the gospel to them."
The note was handed to colleagues on the basis it should be shared publicly if he ever became detained for more than 48 hours.
The senior pastor was among more than 100 people held at the Early Rain Covenant Church in the south-western city of Chengdu, Sichuan province on Sunday.
Wang's elderly mother told the South China Morning Post her son had been arrested "for allegedly inciting subversion of state power".
The letter indicates a strong disdain for the Chinese authorities, with Wang accusing them of "wickedness" in "depriving people of the freedoms of religion and of conscience".
It continued: "As a pastor of a Christian church, I must denounce this wickedness openly and severely.
"The calling that I have received requires me to use non-violent methods to disobey those human laws that disobey the Bible and God. My Saviour Christ also requires me to joyfully bear all costs for disobeying wicked laws.
"But this does not mean that my personal disobedience and the disobedience of the church is in any sense "fighting for rights" or political activism in the form of civil disobedience, because I do not have the intention of changing any institutions or laws of China.
"As a pastor, the only thing I care about is the disruption of man's sinful nature by this faithful disobedience and the testimony it bears for the cross of Christ. "
Wang's congregation had not been registered with the state-sanction Church in the country.
Early Rain Church in #China recently suffered a large scale crackdown: 100 church members were detained by police & the church is still reeling. #Pray for the release of those in detention & for protection and spiritual strength for the whole congregation. pic.twitter.com/M7aKMjEeTS
— CSW_UK (@CSW_UK) December 15, 2018
Among organisations calling for Wang's release is the UK-based anti-religious persecution charity Christian Solidarity Worldwide (CSW).
Speaking to Premier this week, CSW warned of growing persecution against believers in China and a "massive pressure" on unregistered churches wanting to remain independent of state control.
Stay up to date with the latest news stories from a Christian perspective. Sign up to our daily newsletter and receive more stories like this straight to your inbox every morning.