Prophet Muhammad (pbuh) had no motive to invent a religion like Islam
Prophet Muhammad (pbuh) had no reasonable motive to invent a religion like Islam. Some argue that the Prophet (pbuh) invented Islam so that he can have many wives. However, this argument is not convincing since people were already polygamous before Islam was introduced.
Other possible motives argued by critics of Islam are that Muhammad (pbuh) could have done it to or glory. Some even argue that he did it in order to unite the Arabs since they were in conflict at the time and that the Prophet (pbuh) wanted to unite them under one religion. Let’s see if these arguments hold any water. 
Was It For The Money?
When Muhammad (pbuh) was married to Khadijah, he lived a life of relative comfort. However, after his call to Prophethood, his declined dramatically. So it is interesting to note that the Prophet (pbuh) started living simple and poor after his Prophethood and not before.
Umar ibn Al Khattab  states that…
I said: Apostle of Allah, why should I not shed tears? This mat has left its marks on your sides and I do not see in your storeroom (except these few things) that I have seen; Caesar and Chosroes are leading their lives in plenty whereas you are Allah's Messenger. His chosen one, and that is your store! He said: Ibn Khattab, aren't you satisfied that for us (there should be the prosperity) of the Hereafter, and for them (there should be the prosperity of) this world? I said: Yes. 
So here we observe that the Prophet (pbuh) definitely did not live in a palace nor slept on .
People might argue that the Prophet (pbuh) received a through conquests by collecting war booty. However, as we can see the Prophet (pbuh) did not take this money in order to live luxuriously. He used to take that money and give it away in charity instead of keeping it for himself. 
Was It For Glory?
Some critics of Islam may argue that Prophet Muhammad (pbuh) was not interested in amassing wealth, but was interested in attaining veneration from the people so that he can feel good about himself. Let’s see if this is a reasonable conclusion to reach.
During the Prophet’s time, poets were highly respected by the and as we have previously shown, the Qur’an was seen to be unique in its structure. Since the Qur’an was so beautiful in its literary aspect, why would the Prophet (pbuh) give the credit to someone else and not himself if he in fact concocted the book? Poets were praised back then, why didn’t he use the opportunity to receive praise for his masterpiece if he INDEED truly forged it? Even if we were to assume that someone else was assisting him in composing the Qur’an then why didn’t that person use the opportunity to receive praise and acquire fame instead of Prophet Muhammad (pbuh)?
Prophet Muhammad (pbuh) was careful to ensure that Muslims do not exaggerate in praising him…
I heard the Prophet saying, "Do not exaggerate in praising me as the Christians praised the son of Mary, for I am only a Slave. So, call me the Slave of Allah and His Apostle." 
Muslims used to stand up for the Prophet (pbuh) out of respect for him but he prohibited this action from them and told them not to do like the Persians (standing up in respect for someone). 
The Qur'an also corrects Prophet Muhammad (pbuh) on various occasions. At one time in Makkah, Muhammad (pbuh) was busy with a few dignitaries from Quraysh, explaining to them the message of Islam. In the meantime, Ibn Maktoom, a poor , interrupted the Prophet’s conversation unaware that the Prophet (pbuh) was busy with those people. Ibn Maktoom asked him repeatedly to TEACH him some verses of the Qur'an. Prophet Muhammad (pbuh) was not very pleased at this interruption, thus he frowned and turned away from Ibn Maktoom. Several verses were subsequently revealed to Muhammad (pbuh) disapproving of his behavior in this incident. 
It was completely understandable why the Prophet (pbuh) did what he did. He found it more important to try converting the Quraysh leaders to Islam rather than speaking to a man who was already Muslim. He could have spoken to the blind man later for he was busy at that time. If the Prophet (pbuh) authored the Qur’an, why would he criticize himself for mistakes that most, if any people would not realize?
Also a verse in the Qur’an was sent disapproving of the Prophet’s action of praying the funeral prayer for a hypocrite. 
The Prophet (pbuh) also helped his companions in digging a trench by moving sand with them.  This shows that he was not the type of person that would sit back in the shade and let his followers do all the work like all do. 
Now the point to note here is not that the Prophet (pbuh) is not honored by Muslims, for Muslims ask Allah to send His praises and blessings upon the Prophet (pbuh) quite frequently. But the question that one should ask himself is whether the Prophet (pbuh) would fabricate a religion just in order to have people say with their mouths ‘Oh Allah send your praises and blessings upon your Prophet Muhammad’. If a person would want to fabricate a religion he would want the people to mainly to glorify him by the way they TREAThim so that he can be treated well and uniquely. Instead, we don’t see the Prophet (pbuh) ordering the people to treat him this way. Thus, receiving glory and honor are not convincing motives for the Prophet (pbuh) to fabricate the religion of Islam, especially since most of the praise he receives would be received after his death and not during his lifetime.
To Unite The Arabs?
Some critics might want to argue that Prophet Muhammad (pbuh) was a nationalistic person. He did not desire wealth or glory but desired the unification of the Arab world. During the Prophet’s time there were many conflicts taking place between Arab tribes and after the spread of Islam the Arabs were united. Thus, some may see this as a motive for Muhammad (pbuh) forging the religion of Islam.
The disbelievers of Mecca told the Prophet (pbuh) to change the revelation for they didn’t like what it was saying…
And when Our clear revelations are recited unto them, they who look not for the meeting with Us say: Bring a Lecture other than this, or change it. Say (O Muhammad): It is not for me to change it of my accord. I only follow that which is inspired in me. Lo! if I disobey my Lord I fear the retribution of an awful Day. Say: If Allah had so willed I should not have recited it to you nor would He have made it known to you. I dwelt among you a whole lifetime before it (came to me). Have ye then no sense? 
If the Prophet’s goal were to appeal to the Arabs then he would have made the Qur’an say what they wanted to hear. Instead, we notice that the Prophet (pbuh) was honest and was not looking to gain followers or please the Arabs. Rather, he was only interested in fulfilling the commands of Allah by communicating the revelation that He was sending down to him.
Secondly, the Prophet (pbuh) did not believe that being an Arab makes you superior to any another HUMAN being. Rather it was the god-fearing consciousness that one has that makes him superior in the sight of Allah…
Muhammad (Peace and Blessings be upon him) also said: " there is no
excellence for an Arab over a Non-Arab, nor a Non-Arab over an Arab, nor a white person over a black one, nor a black person over a white one, except through piety.” 
Also, why didn’t the Prophet (pbuh) just promote idol worship? It would have made his job easier since most of the Arabs at his time were idol worshippers.
So we note that the idea that as a motive, Prophet Muhammad (pbuh) would falsify Islam in order to unite the Arabs is not a convincing argument.
REFUTING OTHER ARGUMENTS MADE AGAINST THE PROPHET MUHAMMAD (PEACE BE UPON HIM)
To save space and time, I can't list and refute all the arguments made against the Prophet Muhammad (peace be upon him) made by critics of Islam. I would just recommend people go to my website: http://www.answering-christian-claims.com/False_Stories_About_Prophet_Muhammad.html
Several arguments and criticism against the Prophet Muhammad have already been addressed and refuted on my website.
16) I in no way am claiming that the critics of Islam uses these arguments, for I don’t want to attack straw man. However, I think it would be beneficial for me to address these points since it is very likely that the readers themselves who are Islamic critics might possibly believe that Muhammad (pbuh) had some or all of these motives that I am going to mention in order to fabricate the religion of Islam.
17) Arguably, the second greatest companion of the Prophet (pbuh) and is also the second caliph after Abu Bakr.
18) Saheeh Muslim, The Book of DIVORCE, Hadith Number 3507
19) See Saheeh Bukhari, Volume 4, Book of One-fifth of Booty to the Cause of Allah, Hadith number 326, where Umar ibn Al Khattab says “So this property was especially given to Allah's Apostle, but, by Allah, neither did he take possession of it and leave your, nor did he favour himself with it to your exclusion, but he gave it to all of you and distributed it amongst you till this property remained out of it. Allah's Apostle used to spend the yearly expenses of his FAMILY out of this property and used to keep the rest of its revenue to be spent on Allah 's Cause. Allah 's Apostle kept on doing this during all his lifetime.”
20) Saheeh Bukhari, Volume 4, Book of Prophets, Hadith number 654
21) Sunan Al-Tirmidhi, Hadith number 2678 and Sheikh Albani authenticated this hadith in his Jami’ Al Tirmidhi, Hadith number 2754
22) See The Holy Qur’an, Chapter 80, Verses 1-11
23) See The Holy Qur’an, Chapter 9, Verse 84
24) Saheeh Bukhari, Book of Fighting For The Cause of Allah (Jihad), Hadith number 89. Also see Saheeh Muslim, The Book of Jihad and Expedition, Hadith number 4442
25) Since a king would find this degrading to his status and would insist that his designation earns him the right to be glorified and not to be an average worker.
26) The Holy Qur’an, Chapter 10, Verses 15-16
27) Musnad Ahmed, Hadith Number 22391, declared authentic by Shaykh Al Arna’ut
28) He was a Persian and also one of the great companions of the Prophet Muhammad (pbuh)