Chapter fifteen of the Tanya draws a distinction so subtle that most people miss it entirely: the difference between a person who "serves God" and a person who "does not serve Him"—when both are righteous.
The verse in (Malachi 3:18) says: "You shall discern between the righteous and the wicked, between one who serves God and one who does not serve Him." Rabbi Schneur Zalman asks: is "one who serves God" the same as "the righteous"? And is "one who does not serve Him" the same as "the wicked"? No. They are four different categories.
The tzaddik (a righteous person) is called eved Hashem (עבד ה׳)—a "servant of God"—in the past tense. He has already completed his work. He fought the war, transformed his evil into good, and now rests in spiritual peace. He is like a sage who has earned the title through years of study and no longer needs to prove himself.
The benoni who actively struggles against his nature is called oved Elohim (עובד אלוהים)—"one who serves God"—in the present tense. He is in active service right now. The war is ongoing. Every prayer, every moral choice, every resisted temptation is an act of combat.
But then there is the benoni who does not struggle—and this is the surprising category. This person has never sinned. He studies Torah diligently, fulfills all the commandments, and lives a perfectly observant life. But he is not called "one who serves God." He is called "one who does not serve Him." Why? Because his nature does not challenge him. He is naturally studious, naturally calm, naturally disinterested in worldly pleasures. He has no war to fight.
The Tanya's conclusion is radical: spiritual value is measured by effort, not achievement. A person who struggles against a fiery temperament and wins is serving God. A person who is born mild-tempered and never faces temptation may be perfectly righteous—but he is not serving. Service requires resistance. Without an enemy, there is no war. Without a war, there is no warrior.