The Law of Spiritual Dynamics
Newton's "third law" has a spiritual parallel, that I am calling the "law of spiritual dynamics." Romans 7:21 says "So I find it to be a law that when I want to do right, evil lies close at hand."
Whenever I endeavor to do good, whenever I draw close to righteousness, there will be a resistance and opposition. It comes both from within, from my evil flesh, and from the spiritual forces of evil at work in the world. It will often manifest with intensity directly proportionate to the fervor with which I seek to do good.
Thankfully, unlike Newton's third law, which says that in physical matters the reaction will be "equal and opposite," I am comforted in knowing that He who is in me is greater than he who is in the world (1 John 4:4). Nevertheless we must be watchful, aware, and prepared for this reactive opposition.