According to the Islamic faith, invocation in all its forms is a manner of worship:
“And your Lord says, Call upon Me; I will respond to you. Indeed, those who disdain My worship will enter Hell in humiliation!” [Qur’an 40:60]
The Prophet said:
“Dua (invocation / supplication) is the core and essence of worship” [Recorded by Tirmidhi #3371]
The main prerequisite of Islamic monotheism for worshiping Allah is that He is worshiped alone:
“And your Lord has decreed that you worship none but Him…” [Qur’an 17:23]
There are several verses in the Noble Qur’an in this context pointing out the necessity and the duty to worship Allah Alone. Since invocation is a manner of worship; it is therefore not permissible for anybody to invoke anyone and to seek the help of anyone nor plead for assistance from anyone except Allah, as stated in the above verse.
However, in matters pertaining to the daily routine of life and in connection with material things on which every human being has control, a person may seek the help of another person as this is not considered worship. In this context, Allah says (while narrating the story of the Prophet Moses (peace be upon him):
“… The man of his (own) party asked him for help against his foe…” [Qur’an 28:15]
The help and aid which a man seeks from his fellow human beings at times of conflict is like the above said help but seeking help and aid from the dead (whether they were Prophets or pious people), angels, jinns, idols, stones, etc are acts of polytheism. Allah ordered His Prophet Muhammad (ﷺ) to declare:
“I only invoke my Lord and do not associate with Him anyone.” [Qur’an 72:20]
“I hold not for myself (the power of) benefit or harm, except what Allah has willed. And if I knew the unseen, I could have acquired much wealth, and no harm would have touched me. I am not except a warner and a bringer of good tidings to people who believe.” [Qur’an 7:188]
People of the early ages who were indulged in polytheism at times of crisis and hardships worshiped Allah Alone because they knew that Allah Alone can bring salvation to them from their hardships. Qur’an says:
“And when adversity touches you at sea, lost are (all) those you invoke except for Him. But when He delivers you to the land, you turn away (from Him). And man is ever ungrateful.” [Qur’an 17:67]
All disbelievers of the early period did not believe that their gods gave livelihood or do good or harm. They did not deny the existence of Allah as their creator:
“And if you asked them, ‘Who created the heavens and the earth?’ they would surely say, Allah.” [Qur’an 39:38]
Their argument was that they only sought, through such worship, the intercession of those whom they worshiped so that they may take them nearer to Allah. Since this concept of intercession is against the Islamic monotheism, it is condemned in the Qur’an:
“And they worship other than Allah that which neither harms them nor benefits them, and they say, ‘These are our intercessors with Allah’ Say, ‘Do you inform Allah of something He does not know in the heavens or on the earth?’ Exalted is He and high above what they associate with Him” [Qur’an 10:18]
“Unquestionably, for Allah is the pure religion. And those who take protectors besides Him (say), ‘We only worship them that they may bring us nearer to Allah.’ Indeed, Allah will judge between them concerning that over which they differ. Indeed, Allah does not guide he who is a liar and a disbeliever.” [Qur’an 39:3]
So invocation is worship and Allah needs to be worshipped alone. This was the message of all prophets and not just one:
“We had certainly sent Noah to his people, and he said, O my people, worship Allah; you have no deity other than Him.” [Qur’an 7:59]
If anyone else is associated with Allah in His worship then it will be against monotheism and will be considered polytheism. Main problem of the early age polytheists was not “accepting Allah as their God”. Their main issue was to give up the practices and traditions of their forefathers. Referring to the story of ‘Ad in Qur’an, they told the Prophet Hud (peace be upon him):
“Have you come to us that we should worship Allah alone and leave what our fathers have worshipped?” [Qur’an 7:70]
People of the tribe of Quraish when they were called upon by Prophet Muhammad (peace be upon him), to worship Allah Only, replied:
“Has he made the gods (only) one God? Indeed, this is a curious thing.” [Qur’an 38:5]