Any person who abandons the jam’aah of the muslims, severs the safety net Allāh protected him with, has opened the doors to shaytaan and has burdened himself with a great fitnah.

We have seen plentiful examples of this, the end result is consistent, the person abandons the way of correctness and insight, and becomes disgraced, on a path full off errors and contradictions. 

Umar (RadiAllāhu'anhu) made a speech in the city of Jabiyah in which he said: Once Allāh’s Messenger SalAllāhu alayhi wa salam stood up to address us and said: "Whoever hopes for the best of Paradise, should stay with Al Jam’aah. The devil is nearer to a single person than he is to two persons." 

[Ahmad's Musnad 1/1 8, and Tirmidhi, 4/404/21 65.]

Usamah Ibn Sharik (RadiAllāh’anhu) said: I heard Allāh’s Messenger saying: "Allāh’s Hand is with the Jama’ah, and whoever
goes astray the devils will ambush him, just as a wolf ambushes a sheep that goes astray (from the flock).

[Ibn Abu 'Asim 81]