They would be put into room to be sorted between “healthy and able to work” and “undesirable for work”. Those who were healthy and able, would be taken to another room to possibly shower and get their uniforms. Those who were undesired would be taken to gas chambers or would get shot.
Since the flames of the gas chambers and the flames of the battle extinguished his true belief in god, he is saying that he will never forget. He will never forget the pain and suffering and his lost loved ones and all the pain he survived through.