Peter therefore was kept in prison: but prayer was made without ceasing of the church unto God for him.
So Peter was kept in prison, but fervent prayer for him was persistently made to God by the church (assembly).
Peter therefore was kept in the prison: but prayer was made earnestly of the church unto God for him.
While Peter was held in prison, the church offered earnest prayer to God for him.
And so Peter was detained in prison. But prayers were being made without ceasing, by the Church, to God on his behalf.