Stephen Carter from Middle East Concern said although Andrew Brunson's accusations are shocking, formal charges could lead to his freedom.
"He's been held for nearly a year and a half just on the basis of an accusation," he said.
"But now there's a formal charge, a court will have to decide whether the case will proceed to trial.
"It's expected that it will proceed to trial. That does provide an opportunity for acquittal, potentially. That's what we're praying for."
Brunson was jailed in December 2016 over alleged ties with the Fetullah Gulen movement; an organisation authorities believe was connected with a failed coup in Turkey the summer before.
Gulen, who's in the US, is a well-known political opponent of Turkish President Recep Tayyip Erdogan.
After almost a year and a half in jail, Brunson was charged with being a member and leading actor of the terrorist group on Tuesday. He has consistently denied the accusations.
Although Carter remains hopeful for a good outcome he said many political commentators have warned "there's not a great deal of optimism that Turkey is in the mood to do such a deal at the moment".
He added that the pastor's imprisonment isn't a typical case of Christian persecution and noted that the US and Turkey have a strained political relationship.
Carter told Premier: "This is primarily because of the passport in his pocket rather than the cross around his neck.
"In September last year the president of Turkey openly suggested a prisoner swap. He said 'we'll give you Andrew Brunson if you give us Fethullah Gülen.'
"The US government has refused to enter into that negotiation. It does seem to reflect a primarily political difference rather than a specifically religious issue."
Carter has encouraged people to pray for the Church in Turkey as Brunson's imprisonment has weighed heavy on many Christians.
"Turkish Christians are disturbed by this," he said. "It's a part of an ongoing tightening up. Within in society there's an information campaign against Christians in some local media. So many are concerned at this time. The treatment of Andrew Brunson is a particular focus for their concern."
Listen to Stephen Carter speaking with Premier's Cara Bentley here:
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