the first pillar of Islam is the Shahadah - "There is no other God but Allah, and Muhammad is His Messenger." In other words, I cannot accept the monotheistic Islam, without accepting that Muhammad is his one and only prophet, or messenger.
The believer has to admit that Mohammed is God's apostle or messenger; and that he is not His only messenger; the believer has to admit that
God sent a large number of messengers or apostles and the last one of the prophets (coming from
Abraham's progeny) was Mohammed.
And the believer has to obey Mohammed because he is the messenger or apostle of God: Mohammed like the rest of the apostles conveyed the message of God; therefore, it is meaningless that a believer admits that Mohammed is God's apostle then he disobeys this apostle or works according to his own opinion rather than God's instructions.
And the believer has to admit that
Mohammed is the servant and apostle of God; and that Mohammed is a human being like all other men, has the same needs and he cannot avert any harm from himself and from others, and he cannot bring an advantage to himself or to others, unless God Himself has such control of giving the advantage and averting the harm.
(Say [O Mohammed, to them]: "I cannot bring any profit, nor avert any hurt from myself, but only that which God wills.
Had I knowledge of the forefuture, I should have had abundance of wealth, and adversity would not have touched me.
I am only a warner [against idolatry], and a bearer of good tidings [of Paradise] to a people believing.)
The above between brackets is the interpretation by the late Mohammed-Ali Hassan of the Quran 7: 188
قُل لاَّ أَمْلِكُ لِنَفْسِي نَفْعًا وَلاَ ضَرًّا إِلاَّ مَا شَاء اللّهُ وَلَوْ كُنتُ أَعْلَمُ الْغَيْبَ لاَسْتَكْثَرْتُ مِنَ الْخَيْرِ وَمَا مَسَّنِيَ السُّوءُ إِنْ أَنَاْ إِلاَّ نَذِيرٌ وَبَشِيرٌ لِّقَوْمٍ يُؤْمِنُونَ
Moreover, God said in the Quran 72: 21-23
قُلْ إِنِّي لَا أَمْلِكُ لَكُمْ ضَرًّا وَلَا رَشَدًا ..الخ
The explanation:
(Say: "I cannot cause evil for you [as have you demanded for the punishment], nor [can I] make you accept the right guidance [as have I asked of God for you.]"
Say: "None shall defend me against God [if I disobey Him for your sake],
nor can I find [any shelter eventhough] a grave to hide me from Him."
"Except that I [can] deliver what I receive from God and His messages.
And whoso rebels against God and His messenger, for him there awaits the fire of Hell; therein they shall dwell forever [and never get out of it.]"
I do believe that the Almighty will indeed judge me - but my judgement will be based on important things, like how I behaved and how I treated my fellow human beings. Not something as superficial as 'what prophet I did or did not believe in'.
This is not superficial to believe in the apostle that God sent to people or to deny such an apostle; this is a serious thing and has its serious implications.
Moreover, the belief by itself is not enough unless it is confirmed by the righteous work; and it can't be that one says he believes in God alone and in the apostle that God sent, while he works evil and commit the disobedience and contradicts the commandments of God and the instructions of His apostle.
(Whoso obeys the messenger [and is ready to fight in the way of God], obeys God [because the messenger works according to the command of God.]
But whoso turns away [from him and does not fight, We shall punish him;] then We have not sent you [Mohammed] to protect them [from the punishment.])
The above between brackets is the interpretation by the late Mohammed-Ali Hassan of the Quran 4: 80
مَّنْ يُطِعِ الرَّسُولَ فَقَدْ أَطَاعَ اللّهَ وَمَن تَوَلَّى فَمَا أَرْسَلْنَاكَ عَلَيْهِمْ حَفِيظًا