There is a sobriety and firm resolve about life and ministry throughout this chapter. It starts with a soldier illustration and ends with a gentle servant. Standing strong doesn't happen by accident. Like a soldier chooses which things will please his commanding officer, like an athlete learns the rules, like a farmer works hard, that's what it takes to stand firm.
In two or three paragraphs he warns against getting sidetracked by useless arguments, controversies and quarrels. Apparently they have derailed churches for thousands of years!
In two separate instances the character of God is brought in as support, a foundation upon which to stand firm. "If we are faithless, he is faithful; for he cannot deny himself. . . The Lord knows those that are his."
The last paragraph understands opposition to the gospel to be a trap of the devil.