Your companion has neither gone astray nor has erred.
This contains the subject of the oath. This part of the Ayah is the witness that the Messenger of Allah is sane and a follower of Truth. He is neither led astray, such as in the case of the ignorant who does not proceed on any path with knowledge, nor is he one who erred, such as in the case of the knowledgeable, who knows the Truth, yet deviates from it intentionally to something else. Therefore, Allah exonerated His Messenger and his Message from being similar to the misguided ways of the Christians and the erroneous paths of the Jews, such as knowing the Truth and hiding it, while abiding by falsehood. Rather, he, may Allah's peace and blessings be on him, and his glorious Message that Allah has sent him with, are on the perfect straight path, following guidance and what is correct.
Muhammad was sent as a Mercy for all that exists; He does not speak of His Desire
Allah said,
وَمَا يَنطِقُ عَنِ الْهَوَى