The opposite of Tawheed is to give one or more of the rights of Allah to someone or something else. This is called shirk. There is a category of shirk for every category of tawheed, and an example of shirk for every example of tawheed.

Shirk will not be forgiven if a person does not repent for it in this life.

{ إِنَّ ٱللَّهَ لَا يَغۡفِرُ أَن يُشۡرَكَ بِهِۦ وَيَغۡفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَآءُۚ وَمَن يُشۡرِكۡ بِٱللَّهِ فَقَدِ ٱفۡتَرَىٰٓ إِثۡمًا عَظِيمًا }
[Surah An-Nisāʾ: 48]

• Muhsin Khan and Taqi-ud-Din al-Hilali:
  “Verily, Allah forgives not that partners should be set up with him in worship, but He forgives except that (anything else) to whom He pleases, and whoever sets up partners with Allah in worship, he has indeed invented a tremendous sin.”
  An-Nisa’, Ayah 48

{ إِنَّ ٱللَّهَ لَا يَغۡفِرُ أَن يُشۡرَكَ بِهِۦ وَيَغۡفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَآءُۚ وَمَن يُشۡرِكۡ بِٱللَّهِ فَقَدۡ ضَلَّ ضَلَٰلَۢا بَعِيدًا }
[Surah An-Nisāʾ: 116]

• Muhsin Khan and Taqi-ud-Din al-Hilali:
  “Verily! Allah forgives not (the sin of) setting up partners in worship with Him, but He forgives whom he pleases sins other than that, and whoever sets up partners in worship with Allah, has indeed strayed far away.”
  An-Nisa’, Ayah 116

It causes the person to reside in the Hellfire forever.
It should also be noted that kufr (disbelief) is slightly more comprehensive than shirk; you can commit kufr without shirk, but every type of major shirk is kufr.

Finally, not everyone who commits kufr is necessarily a kaafir (disbeliever). Certain conditions have to be present, and certain things can stop that from taking place.

Sources:

Lecture Notes:
https://www.digitalmimbar.com/AqeedahNotesTimHumble.pdf