According to the anti-persecution charity Christian Solidarity Worldwide (CSW), Talahon Nigosi Kassa Ratta wasn't allowed access to his family or solicitors for the majority of his detention
Mr Ratta, who was released on Tuesday, was a member of the Khartoum Bahri Evangelical Church.
A pastor from the Sudan Church of Christ, Revd Hassan Abdulraheem, was arrested shortly after Mr Ratta last December and still has not been released.
CSW has said his lawyers have been informed that he is being held on suspicion of security crimes, which carry the death penalty in Sudan.
Meanwhile, an committee sanctioned by the Sudanese government has been illegally selling off land and buildings belonging to the Sudan Evangelical Presbyterian Church to Muslim businessmen.
This is despite an official court order recognising the committee as breaking the law.
Rafat Obid, a senior member of the Sudan Evangelical Presbyterian Church who the church elected to be in charge of it's land and buildings, was arrested in May charged with impersonation, forgery and criminal misappropriation.
He's been bailed and is awaiting trial.
CSW's Chief Executive Mervyn Thomas said: "We welcome the decision to release Mr Ratta, however, we continue to be concerned by his prolonged detention without charge.
"Mr Ratta's case shows the power that NISS wields within Sudan to detain private individuals without any legal recourse, which it is currently using to harass and detain members of the Christian community and civil society.
"We are calling for a review of the agency's powers of arrest and detention and for the criminal proceedings against Rev. Abdulraheem and Mr Rafat Obid, which have no basis, to be dropped.
"Furthermore we call on the Ministry of Religious Endowments and Guidance to overturn its decision to recognise the illegally-convened committee of the Sudan Evangelical Presbyterian Church and to end its interference in the church's affairs."