Persisting, and insisting on truth and divine Truth's power enables us to overcome any obstical.When we find ourselves feeling unwell, sometimes it seems sudden and other times it seems to sneak up. But by watching thought and through insistence on truth—about yourself as Spirit’s (God) spiritual reflection—this is a powerful antidote to illness. Arguing mentally for yourself, only on the side of divine Truth, as being healthy and good, even when you’re on top of your game, is a powerful prophylactic. And it’s good practice.

For example: while running recently, I suddenly felt pain. And, the other day, while hanging with my family over Christmas, I felt a tempting feeling that implied a sore throat was en route. What did I do in both of these cases? I arrested those thoughts and feelings and stopped them in their tracks. I refused their entrance into my consciousness. Instead, I insisted on Truth’s power to keep me safe, and held to the fact that health was/is a permanent, spiritual fact about who I am as God’s image and likeness, or reflection—as taught in Christian Science.

Be sober, be vigilant; because your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour: Whom resist stedfast in the faith, knowing that the same afflictions are accomplished in your brethren that are in the world.
—I Peter 5:8, 9

Persist and insist from the basis of divine Truth

In both cases stated above I needed to persist. I had to reason from the basis of divine Truth—God’s eternal, infinite goodness and that I/we reflect God’s goodness. Christian Science defines God’s law of harmony as divine Principle, and this law supports and supplies every spiritual fact and quality that makes us who we are as God’s spiritual and healthy ideas. In both cases, by persisting from the basis of divine Truth, these tempting thoughts and feelings immediately vanished. No pain, no tempting feeling of illness lingered. They just ended.

I have found over and over it’s not enough to just mentally deny illness. Rather, I find it’s important to prayerfully persist and insist on what is true — the spiritual facts of health, harmony, life—while also denying what is false (sickness, pain, discomfort). Taking time to deny sickness and sin, while also affirming what’s true—goodness, health, purity—is part of Christian Science treatment, as explained in the chapter “Christian Science Practice” in Science and Health with Key to the Scriptures by Mary Baker Eddy.

By lifting thought above error, or disease, and contending persistently for truth, you destroy error.
—Mary Baker Eddy, Science and Health, p.400:18

Assert Truth and practice it

You can do this, too! To start putting divine Truth into practice, I recommend the purchase and reading of Science and Health by Mary Baker Eddy — as the discoverer of Christian Science, she explains divine Science best.

Mentally insist that harmony is the fact, and that sickness is a temporal dream. Realize the presence of health and the fact of harmonious being, until the body corresponds with the normal conditions of health and harmony.
—Mary Baker Eddy, Science and Health, p.412