The word has the literal meaning of a message-bearer or agent. This explains the object of the creation of angels. They bring messages of God and execute His will in the universe. Thus angels are a part of the physical and the spiritual worlds.
In the spiritual worlds the influence of the angels is direct and works without any intervening agency. Disbelief in angels would therefore mean shutting off the avenue through which the light of God comes to man.
35: 2. All praise belongs to ALLAH, the originator of the heavens and the earth, Who employs the angels as Messengers, having wings, two, three, and four. HE adds to HIS creation whatever HE pleases; for ALLAH has power over all things.
2: 98. Say `Whoever is an enemy to Gabriel’ – for he it is who has caused it to descend on thy heart by the command of ALLAH, fulfilling that revelation which precedes it, and is a guidance and glad tidings to the believers.
2: 99. `Whoever is an enemy to ALLAH, and HIS angels, and HIS Messengers, and Gabriel, and Michael, then surely, ALLAH is an enemy to such disbelievers.’
2:178. It is not righteousness that you turn your faces to the East or the West, but truly righteous is he who believes in ALLAH and the Last Day and the angels and the Book and the Prophets, and spends his money out of love for HIM, on the kindred and the orphans and the needy and the wayfarer and those who ask for charity and for ransoming the captives; and observes prayer and pays the Zakat; and those who fulfil their promise when they have made one, and the patient in poverty and afflictions and the steadfast in the time of war; it is these who have proved truthful and it is these who are truly God-Fearing.
2:286. This Messenger of OURS believes in that which has been revealed to him from his Lord, and so do the believers; all of them believe in ALLAH, and in HIS angels, and in HIS Books, and in HIS Messengers, saying, `We make no distinction between any of HIS Messengers;’ and they say `We have heard and we are obedient. Our Lord, we implore THY forgiveness, and to Thee is the returning.’
22: 76. ALLAH chooses HIS Messengers from among angels and from among men. Surely, ALLAH is All-Hearing, All-Seeing.
4:137. O ye who believe ! believe in ALLAH and HIS Messenger and in the Book which HE had revealed to HIS Messenger, and the Book which HE revealed before it. And whoso disbelieves in ALLAH and HIS angels, and HIS Books and HIS Messengers and the Last Day, has surely strayed far away.