I had the same doubt years ago which has now completely vanished since I have completely realized the answer which very less people today know and as a result, engages in useless ignorant arguments. Pls see the accurate answer below:
According to the classification given in Padma Purana, Uttara Khanda (236.18-21), Hindu Puranas have been classified in accordance with the three gunas or qualities as Sattva (Truth and Purity), Rajas (Dimness and Passion) and Tamas (Darkness and Ignorance).
Hence Shri VedaVyasa (partial incarnation of Supreme God Vishnu), while composing these 18 Mahapuranas, has himself categorized them into three types viz. Satvika Puranas, Rajasa Puranas and Tamasa Puranas. While the Satvic puranas are verily valid, Rajasic puranas have some misleading statements also. The Tamasic puranas are predominant with misleading statements and are intended to mislead the ineligible from the truth.
The divisions of the eighteen Puranas is defined by Lord Siva to Goddess Parvati as classification into Satvika, Rajasa and Tamasa in Padmapurana Uttarakhanda 235 Adhyaya.
1) Satvika Puranas (Authentic & True Contents):
वैष्णवं नारदीयं च तथा भागवतं शुभम् ।
गारुडं च तथा पाद्मं वाराहं शुभदर्शने ।
सात्विकानि पुराणानि विज्ञेयानि शुभानि वै ।।
· Vishnu Purana
· Naradiya Purana
· Padma Purana
· Garuda Purana
· Varaha Purana
· Srimad Bhagavata Maha Purana
2) Rajasa Puranas (Truth & Misleading Mixed):
ब्रह्माण्डं ब्रह्मवैवर्तं मार्कण्डेयं तथैव च ।
भविष्यं वामनं ब्राह्मं राजसानि निबोधत ।।
· Brahmanda Purana
· Brahmavaivarta
· Markandeya Purana
· Bhavishya Purana
· Vamana Purana
· Brahma Purana
3) Tamasa Puranas (Mostly Misleading Contents, Very less truth):
मात्सयं कौर्मं तथा लैङ्गम् शैवं स्कान्दं तथैव च ।
आग्नेयं च षडेतानि तामसानि निबोधत ।।
· Matsya Purana
· Kurma Purana
· Linga Purana
· Shiva Purana
· Skanda Purana
· Agni Purana
Pls note that:
सात्विका मोक्षदाः प्रोक्ताः राजसा सर्वदा अशुभाः ।
तथैव तामसा देवि निरयप्राप्तिहेतवः ।। (प.पु. उत्तर 236.21)
Among these puranas, Satvika Puranas lead one to liberation, Rajasa do not cause good and tamasa puranas lead one to hell.
Having said this, not all portions of the Rajasa and Tamasa Puranas are invalid. Wherever they are in accordance with the satvika puranas they are perfectly valid and reliable.
One should bow down to Shri VedaVyasa who has made sure that every body achieves their nature (svabhava) by reading the respective puranas.
Since Supreme God Shri Hari (Vishnu) is impartial he has made sure that everybody is empowered in reaching their respective states.