وَمَن يُشَاقِقِ الرَّسُولَ مِن بَعْدِ مَا تَبَيَّنَ لَهُ الْهُدَى
And whoever contradicts and opposes the Messenger after the right path has been shown clearly to him.
refers to whoever intentionally takes a path other than the path of the Law revealed to the Messenger, after the truth has been made clear, apparent and plain to him.
Allah's statement,
وَيَتَّبِعْ غَيْرَ سَبِيلِ الْمُوْمِنِينَ
and follows other than the believers' way,
refers to a type of conduct that is closely related to contradicting the Messenger. This contradiction could be in the form of contradicting a text (from the Qur'an or Sunnah) or contradicting what the Ummah of Muhammad has agreed on. The Ummah of Muhammad is immune from error when they all agree on something, a miracle that serves to increase their honor, due to the greatness of their Prophet.
There are many authentic Hadiths on this subject.
Allah warned against the evil of contradicting the Prophet and his Ummah, when He said,
نُوَلِّهِ مَا تَوَلَّى وَنُصْلِهِ جَهَنَّمَ وَسَاءتْ مَصِيرًا
We shall keep him in the path he has chosen, and burn him in Hell --- what an evil destination!
meaning, when one goes on this wicked path, We will punish him by making the evil path appear good in his heart, and will beautify it for him so that he is tempted further.
For instance, Allah said,
فَذَرْنِى وَمَن يُكَذِّبُ بِهَـذَا الْحَدِيثِ سَنَسْتَدْرِجُهُمْ مِّنْ حَيْثُ لَا يَعْلَمُونَ
Then leave Me Alone with such as belie this Qur'an. We shall punish them gradually from directions they perceive not. (68;44)
فَلَمَّا زَاغُواْ أَزَاغَ اللَّهُ قُلُوبَهُمْ
So when they turned away (from the path of Allah), Allah turned their hearts away. (61;5)
and,
وَنَذَرُهُمْ فِى طُغْيَانِهِمْ يَعْمَهُونَ
And We shall leave them in their trespass to wander blindly. (6;110)
Allah made the Fire the destination of such people in the Hereafter. Indeed, the path of those who avoid the right guidance will only lead to the Fire on the Day of Resurrection, as evident by Allah's statements,
احْشُرُواْ الَّذِينَ ظَلَمُواْ وَأَزْوَجَهُمْ
(It will be said to the angels);"Assemble those who did wrong, together with their companions (from the devils). (37;22)
and,
وَرَأَى الْمُجْرِمُونَ النَّارَ فَظَنُّواْ أَنَّهُمْ مُّوَاقِعُوهَا وَلَمْ يَجِدُواْ عَنْهَا مَصْرِفًا
And the criminals, shall see the Fire and apprehend that they have to fall therein. And they will find no way of escape from there. (18;53)