You’re going along, just minding your own business, doing what you think God wants you to do, trying to be an obedient follower of Christ, when here comes this thought that opposes God’s will and authority. Here comes an obstacle in your path. Here comes a person to discourage and deflate you.
At this point, many suffer unnecessary defeat by being surprised that such an occurrence would happen to them. Or simply by being unfaithful and succumbing.
The Christian life is not a playground, it’s a battleground. We have every reason to be confident that “He who is in us is greater than he who is in the world” (1 John 4:4). But that does not spare us from having to engage in the battle. Victory is ours. Immunity is not.