Say 'there is no god but Allah', and I will testify for you by it on the Day of Judgment

The Prophet (may Allah’s peace and blessings be upon him) asked his uncle Abu Tālib, while in the throes of death, to utter the testimony of faith, so that he could intercede for him with it on the Day of Judgment and testify that he was a Muslim. But he refused to utter it, fearing that the Quraysh would revile him and say that he embraced Islam out of fear of death and weakness!

He said to the Prophet (may Allah’s peace and blessings be upon him): Were it not for that, I would have made you joyful by saying the testimony, fulfilling your wish and making you pleased.

So, Allah Almighty revealed a verse indicating that the Prophet (may Allah’s peace and blessings be upon him) cannot guide people in the sense of inspiring them to accept Islam, but it is Allah Almighty alone Who grants such guidance to whomever He wills; and that the Prophet (may Allah's peace and blessings be upon him) guided them by clarifying, showing, and calling them to the straight path.