One has to approach a bona fide spiritual master to receive the knowledge. Such a spiritual master should be accepted in full surrender, and one should serve the spiritual master like a menial servant, without false prestige. Satisfaction of the self-realized spiritual master is the secret of advancement in spiritual life. Inquiries and submission constitute the proper combination for spiritual understanding.
(Bhagavad-Gita----4:34----purport).

Unless there is submission and service, inquiries from the learned spiritual master will not be effective. One must be able to pass the test of the spiritual master, and when he sees the genuine desire of the disciple, he automatically blesses the disciple with genuine spiritual understanding. In this verse, both blind following and absurd inquiries are condemned.
(Bhagavad-Gita----4:34----purport).

When we speak of transcendental knowledge, we do so in terms of spiritual understanding. As such, there is nothing so sublime and pure as transcendental knowledge. Ignorance is the cause of our bondage, and knowledge is the cause of our liberation. This knowledge is the mature fruit of devotional service, and when one is situated in transcendental knowledge, he need not search for peace elsewhere, for he enjoys peace within himself.
(Bhagavad-Gita----4:38----purport).

The living entity, by accepting his material existence, has become situated differently than in his spiritual existence. But if one understands that the Supreme is situated in His Paramātmā manifestation everywhere, that is, if one can see the presence of the Supreme Personality of Godhead in every living thing, he does not degrade himself by a destructive mentality, and he therefore gradually advances to the spiritual world. The mind is generally addicted to sense gratifying processes; but when the mind turns to the Supersoul, one becomes advanced in spiritual understanding.
(Bhagavad-Gita----13:29----purport).

Devotional service is a process of spiritual understanding. Wherever devotional service exists, the material contamination cannot coexist. Devotional service to the Lord and the Lord Himself are one and the same because they are spiritual; devotional service takes place within the internal energy of the Supreme Lord.
(Bhagavad-Gita----15:20----purport).

For a devotee, there is no point in making friendships with ordinary persons; he should make friendship with other devotees so that by discussing among themselves, they may elevate one another on the path of spiritual understanding. This is called iṣṭa-goṣṭhī.
(Srimad Bhagavatam----3:29:17---purport).

To be too fat is not very good for spiritually advanced life. Rather, one should reduce because if one becomes fat it is an impediment to progress in spiritual understanding. One should be careful not to eat too much, sleep too much or remain in a comfortable position. Voluntarily accepting some penances and difficulties, one should take less food and less sleep.
(Srimad Bhagavatam----3:33:14----purport).

A person desiring to advance in spiritual understanding should be extremely careful to avoid reading ordinary literature. The world is full of ordinary literature that creates unnecessary agitation in the mind. Such literature, including newspapers, dramas, novels and magazines, is factually not meant for advancement in spiritual knowledge.
(Srimad Bhagavatam----7:13:7---purport).

The living entity is a spiritual soul, and the material body is his encagement. This is the beginning of spiritual understanding.
(Srimad Bhagavatam----9:19:20----purport).