Siege!

You cannot make use of implements of evil. No matter how effective they may be, you know in your heart that it will not end well. You execute the heretic yourself in front of a large crowd. You are met with cheers followed by a great silence. That silence is an uncertainty for how this conflict will end. You return to the old way of battle and prepare every able bodied man and woman for war. If God is on your side, you shall win, and if he is not you are all doomed anyway.
You order the greatest attack the world has ever seen, troops charging with ladders, grappling hooks, and siege towers. Every able bodied man and even some women join in the attack, eager to do the bidding of their creator. You take a count before the attack, over fifty thousand troops of assembled. Some are heavily armored and wield weapons of masterwork quality while others where simply clothes and are armed with a pitchfork. Everyone is ready to die for the cause and you must outnumber the forest people several hundred to one. How could you lose?
Once everyone is assembled and armed with the best weapons available you order an all out charge. You lead the charge yourself. No point in holding back, if this fails your life will not last long any way you look at it. The army charges and a cacophony of battle calls fill the air. It is strange, no arrows fly when you enter range. Not until the armies are much closer do arrows fly. A wave of arrows are released from the fortress perfectly synchronized, as if they were all released by the same being. The entire front line falls simultaneously. Your troops simply run over the battles so filled with fervor. They are true zealots. The second, third, fourth and fifth lines are knocked down as well before your troops finally reach the walls. Immediately they try to scale and are served up more arrows and boiling water. The few soldiers that do manage to make it up the wall are chopped to pieces by warriors waiting at the top. Ladders are pushed off, siege towers are burned before they can ever reach the fortress and grappling hooks are simply cut. The majority of the troops are left in disarray at the base of the fortress with no way to get up. The ladders are pushed down long before anyone can climb to the top and your archers are simply ineffective. They hit their allies more frequently than their enemies. You know you have to call for a retreat, the battle is lost, and God does not favor you. Men are beginning to claw at the gate and simply bash the wall with their weapon as if they believed they could do some damage. You blow a horn and order a retreat. As troops begin to leave the wall they are mowed down by arrows. Very few manage to escape, and they leave behind a writhing mass of the dying and the dead. Thousands have been lost and no progress has been made. You do not know if a single forest person was killed in the assault, and the fortress stands, strong as ever.
Knowing that God has abandoned you, you do the honorable thing and take your own life. Without leadership the army disbands. The people's faith in the church is permanently shaken and revolution inevitably occurs. The forest people spread outwards from the fortress and are able to permanently establish a presence in the civilized territories with no empire to stop them. They have won.
End Of Story