“Simply by associating with exalted devotees, anyone can attain perfection of knowledge and with the sword of knowledge can cut to pieces the illusory associations within this material world. Through the association of devotees, one can engage in the service of the Lord by hearing and chanting [śravaṇaṁ kīrtanam]. Thus one can revive his dormant Kṛṣṇa consciousness and, sticking to the cultivation of Kṛṣṇa consciousness, return home, back to Godhead, even in this life.” (Shrimad Bhagavatam 5.12.16)

Associating with genuine devotees (very few in numbers)
                                                ⇓   (Leads to)
Become a descent human being (Truthful, compassionate, charitable, clean, simple)

Based on Shrimad-Bhagavatam, the above 5 qualities are the pillars of Sanatan Dharma. Thus what makes a person religious. Only those who have developed these qualities to some degree can be considered pure.

Only a pure person becomes eligible to know God and enter the kingdom of God.

Not following the pillars of Sanatan Dharma and going to temples, chanting mantras, and performing rituals does NOT make one religious nor indicate purity.