The reason for the revelation of this Surah has already been mentioned.
Ikrimah said,
"When
- the Jews said, `We worship Uzayr, the son of Allah,' and
- the Christians said, `We worship the Messiah (Isa), the son of Allah,' and
- the Zoroastrians said, `We worship the sun and the moon,' and
- the idolators said, `We worship idols,'
Allah revealed to His Messenger,
قُلْ هُوَ اللَّهُ أَحَدٌ
Say;"He is Allah, One."
meaning,
- He is the One, the Singular,
- Who has no peer,
- no assistant, no rival,
- no equal and none comparable to Him.
This word (Al-Ahad) cannot be used for anyone in affirmation except Allah the Mighty and Majestic, because He is perfect in all of His attributes and actions.
Concerning His saying,
اللَّهُ الصَّمَدُ