CSW has called for the release of Rev Hassan Abduraheem, Rev Kuwa Shamal, Petr Jašek and Abdumonem Abdumawla who were arrested in December 2015 and have been in detention since May 2016.
The case against the men centres on the provision of finances for the medical treatment of a young man from Darfur who was injured in a demonstration in 2013.
Population: 36 million
Christian population: 1.9 million
The government of Sudan implements a one-religion policy (Islam)
Pastors and Christians face imprisonment if they share their beliefs and churches are often attacked and destroyed
Jašek, who is a Czech national, travelled to Khartoum in December 2015 to meet the young man and donated almost £4,000 toward his treatment.
Rev Abduraheem, Rev Kuwa Shamal and Mr Abdumawla, had helped to arrange the visit.
The four men are charged with seven crimes including espionage and waging war against the state.
The state claim the £4,000 contribution was not for medical treatment but to fund rebel fighting.
If the group are found guilty, they could receive the death penalty.
CSW have claimed that since his arrest, Jašek has had limited contact with his family and legal representative while Rev Abduraheem was not permitted visits from family members until May 2016.
CSW's Chief Executive Mervyn Thomas said: "This month marks one year since 4 men were arrested in Sudan on grave charges which they have denied, but which have been brought against them simply for extending compassion to a man in need of medical treatment.
"In Rev Shamal's case, being a senior Christian leader and a member of the Nuba ethnic groups is sufficient for him to be targeted by the security services.
"We urge the Sudanese authorities to immediately and unconditionally release these men and to drop all charges against them."