Welcome to the true Church Militant, Praetor
The sword of an angel can cut any substance, not because it is sharp, but because it is more real than any substance. Their goals are not our goals, and their judgement may be upon you at inopportune times. You can fight a demon, but always run from an angel. Remember the danger of the power of God.
Demons "exist" second-hand, manifesting in reflections, shadows, windblown dust, and of course, through possession. They enter the world through the corridor formed by two mirrors
reflecting each other, from ponds still and clear as glass, and when
someone dreams the right dreams.
The presence of a demon degrades reality. Possessed individuals often manifest "magical" abilities, and will slowly become more and more monstrous. Every demon or case of possession you will encounter will be unique; treat every encounter as though it was your first, and expect the unexpected.
I will see you in the field.