In my conclusion, Prophet Muhammad either truly believed he was speaking to God, or he simply was someone who wanted to renew the religion of Abraham (Ibrahim) and lied to convince people to be good. He hated all forms of idolatry, including Christianity.
Renewer of the religion of Abraham
There were others who claimed to follow the religion of Ibrahim that lived during the life of Prophet Muhammad or in the same era, such as Zaid bin Amr, who also used to abhor the practices of the arabs – idolatry, killing the infant girls, etc..
Narrated Asma bint Abi Bakr:
I saw Zaid bin Amr bin Nufail standing with his back against the Ka’ba and saying, “O people of Quraish! By Allah, none amongst you is on the religion of Abraham except me.” He used to preserve the lives of little girls: If somebody wanted to kill his daughter he would say to him, “Do not kill her for I will feed her on your behalf.” So he would take her, and when she grew up nicely, he would say to her father, “Now if you want her, I will give her to you, and if you wish, I will feed her on your behalf.”
Narrated Jabir bin `Abdullah:The Prophet (ﷺ) said, “My similitude in comparison with the other prophets is that of a man who has built a house completely and excellently except for a place of one brick. When the people enter the house, they admire its beauty and say: ‘But for the place of this brick (how splendid the house will be)!”
What did he gain?
First of all, financial benefits.
1/5th of all the war booty would directly go to Prophet Muhammad. Sunnis don’t still follow this, but take a look at this Shia article describing Khums as 20% that goes directly to the war booty.
Second of all, sexual benefits:
He was able to use God as a moral justification to do whatever pleased him. For example being unfair to his wife Sauda and giving her night to Aisha instead. Aisha actually said , “I feel that your Lord hastens in fulfilling your wishes and desires.” (Bukhari and Muslim) when this happened. For example marrying more than the 4 women he allowed other men to marry.
For example allowing himself to sleep with their female captives of war and saying God was his justification. Prophet Muhammad himself owned many slaves, 16 male slaves and his female slaves included: Salma, Maymoonah bint Sa’d; Khadrah; Radwa; Razeenah; Umm Dameerah; Maymoonah bint Abi ‘Usayb; Maariyah and Rayhaanah. The story of Rayhaanah:
Umar Ibn al-Hakam narrated: “The Messenger of God freed Rayhana Bint Zaid Ibn`Amr Ibn Khunaqa. She had been married to a loving and honouring husband. She said, ‘I shall never have a husband after him.’ She was beautiful. When Banu Quraiza were taken captive, the booty was shown to the Messenger of God, and she was among the captives that were shown him so he commanded her to be set aside.He used to have a choice from each booty. She herself narrated, ‘When I was set aside, he chose me and sent me to the house of Umm al-Mundsir Bint Qais for some days till he had the captives killed and the spoils distributed.
He was so attracted by her that he traded seven other slaves for her:
Anas said: A beautiful slave-girl fell to Dihyah. The Apostle of Allah purchased her for seven slaves. (Abu Dawud)
Or Juwayriyyah, Aisha saw her that she was beautiful and attractive, and she got worried because she knew Prophet Muhammad would be attracted to her:
Narrated Aisha, Ummul Mu’minin:
Juwayriyyah, daughter of al-Harith ibn al-Mustaliq, fell to the lot of Thabit ibn Qays ibn Shammas, or to her cousin. She entered into an agreement to purchase her freedom. She was a very beautiful woman, most attractive to the eye.
Aisha said: She then came to the Apostle of Allah (peace_be_upon_him) asking him for the purchase of her freedom. When she was standing at the door, I looked at her with disapproval. I realised that the Apostle of Allah (peace_be_upon_him) would look at her in the same way that I had looked. (Abu Dawud)
Third of all, respect fame and honour more than any other king
His companions respected and deified him to the point that nobody else was respected
At the treaty of Al-Hudaibiya in which Urwa came to the Prophet (SAW) to negotiate its terms and when returned to his people and said, “O people! By Allah, I have been to the kings and to Caesar, Khosrau and An-Najashi, yet I have never seen any of them respected by his courtiers as much as Muhammad is respected by his companions. By Allah, if he spat, the spittle would fall in the hand of one of them (i.e. Sahaba) who would rub it on his face and skin; if he ordered them, they would carry out his order immediately; if he performed ablution, they would struggle to take the remaining water; and when they spoke, they would lower their voices and would not look at his face constantly out of respect.”
After he (i.e. Allah’s Apostle) had performed ablution, the remaining water was taken by the people and they started smearing their bodies with it (as a blessed thing).
Narated By Ibn Shihab : Mahmud bin Ar-Rabi’ who was the person on whose face the Prophet had ejected a mouthful of water from his family’s well while he was a boy, and ‘Urwa (on the authority of Al-Miswar and others) who testified each other, said, “Whenever the Prophet , performed ablution, his companions were nearly fighting for the remains of the water.”
He actually commanded them that God ordered them to love him:
“None of you will be a true believer until I am more beloved to you than yourself”. [Bukhari]
And in the Quran he is praised as being of the perfection of character:
The Prophet is more worthy of the believers than themselves, and his wives are [in the position of] their mothers. (33:6)
And indeed, you are of a great moral character. (68:4)
And whenever he had an issue, he had God come to his defense and straighten out the problem. Like when his wives were not happy (despite him being the perfect husband any woman would ever want), God comes and threatens his wife that they will be replaced with better women:
Perhaps his Lord, if he divorced you [all], would substitute for him wives better than you – submitting [to Allah ], believing, devoutly obedient, repentant, worshipping, and traveling – [ones] previously married and virgins. (66:4)
Once Umar was walking with the Prophet ﷺ, and he said
“By Allah, I love you O Prophet of Allah”.
The Prophet ﷺ asked, “More than your children, O Umar?”
Umar (ra) replied “Yes, O Prophet of Allah.”
The Prophet ﷺ asked, “More than your money, O Umar?”
And Umar (ra) replied, “Yes, O Prophet of Allah”
The Prophet ﷺ then asked him, “More than yourself, O Umar?”
Umar (ra) replied, “No, O Prophet of Allah”
The Prophet ﷺ then told him “O Umar, your faith will never be complete until you love me more than yourself.”
We can see that there is a clear motive here. He is receiving massive benefits from being a Prophet of God.
Read more at Muhammad’s Just In Time Revelations