To answer your question, it is important to distinguish between two principal approaches criticized in the materials: serving through unconditional obedience and acting self-proclaimed in one’s own name.

Divine Service vs. Self-Glorification: Unmasking True Allegiance

To answer your question, it is important to distinguish between two principal approaches criticized in the materials: serving through unconditional obedience and acting self-proclaimed in one’s own name.

According to the quote from file link txt, a true messenger of God acts in the name of the Sender—that is, he is guided by God’s will—whereas if a person acts solely in his own name, he is a messenger of the devil.

As it is stated: "Who is this messenger who acts in his own name?

If he is a messenger, an apostle, a clergyman, then he must act not in his own name, but in the name of the one who sends him, calls him, dedicates him. But if he acts in his own name, he is not a messenger. ... But the devil combines them. He sends his messengers into the world, strengthens them... Antichrists are messengers of the devil, acting in their own name." (source: link txt)

Similarly, the same source notes that the true apostles of Christ act in the name of their Teacher, whereas the devil’s messengers act in their own name. This emphasizes that the main characteristic of serving God is self-denial and the absence of a desire for personal glory or power.
"Apostles of Christ are helpers of God. Meanwhile, the devil becomes an assistant to his messengers. The apostles of Christ act in the name of their Teacher. The messengers of the devil act in their own name." (source: link txt)

Furthermore, the material from file link txt highlights that true service to God cannot be separated from virtue. A person devoid of the pursuit of virtue and committing wicked deeds—while quietly convinced of his own righteous service—is actually acting in the name of the devil.
"Thus, even self-styled Christians err, imagining that without virtue they serve God. No! They serve not God, but the devil." (source: link txt)
and also:
"He who serves virtue is a slave of God, and he who serves sin is a slave to sin and, consequently, a slave to the culprit of evil—the devil." (source: link txt)

Thus, to determine which Christians truly serve God and which may be acting in the name of the devil, it is necessary to evaluate both the source and sincerity of their motivation as well as the presence or absence of virtuous behavior in their actions. If the deeds are aimed solely at personal glory and self-affirmation rather than serving God, and if these deeds are accompanied by sinfulness and a lack of moral foundation, these are indications that such actions are aligned with forces opposed to those of God.

Supporting citation(s):
"Who is this messenger who acts in his own name? If he is a messenger, an apostle, a clergyman, then he must act not in his own name, but in the name of the one who sends him, calls him, dedicates him. But if he acts in his own name, he is not a messenger. ... Antichrists are messengers of the devil, acting in their own name." (source: link txt)

"Apostles of Christ are helpers of God. Meanwhile, the devil becomes an assistant to his messengers. The apostles of Christ act in the name of their Teacher. The messengers of the devil act in their own name." (source: link txt)

"Thus, even self-styled Christians err, imagining that without virtue they serve God. No! They serve not God, but the devil." (source: link txt)

"He who serves virtue is a slave of God, and he who serves sin is a slave to sin and, consequently, a slave to the culprit of evil—the devil." (source: link txt)