The Prophet is a light in terms of his message and guidance
Allah bestowed the light of the Message of Islam and the true guidance on the Prophet (peace be upon him), wherewith Allah guides the hearts of those whom He wills of His servants. Thus, undoubtedly Allah is the source of the Message of Islam and the One Who made the Prophet a source of guidance to others. Allah (Exalted be He) says:
It is not given to any human being that Allâh should speak to him unless (it be) by Revelation, or from behind a veil, or (that) He sends a Messenger to reveal what He wills by His Leave. Verily, He is Most High, Most Wise.
And thus We have sent to you (O Muhammad صلى الله عليه و سلم) Ruh (a Revelation, and a Mercy) of Our Command. You knew not what is the Book, nor what is Faith? But We have made it (this Qur’ân) a light wherewith We guide whosoever of Our slaves We will. And verily, you (O Muhammad صلى الله عليه و سلم) are indeed guiding (mankind) to the Straight Path (i.e. Allâh’s Religion of Islâmic Monotheism).
The Path of Allâh to Whom belongs all that is in the heavens and all that is in the earth. Verily, all matters at the end go to Allâh (for decision).
This light does not come from the seal of the Prophets as claimed by some atheists. The Prophet had a natural human body composed of blood, flesh and bones ...Etc. He had a natural birth which happened through the natural contact between a father and a mother. He was not created before that time. No authentic report is related by the Prophet (peace be upon him) which indicates that his light was the first thing Allah created or that Allah (Part No. 1; Page No. 447) grasped a handful of the light of His face out of which Prophet Muhammad was created and that when Allah looked at that handful of light, it started to sprinkle in the form of drops; out of each drop a Prophet was created or that all human beings are created from the Prophet's light (Page: 366 and following pages from the book entitled "Majmu` Al-Fatawa", by
Ibn Taymiyyah, vol. 18).