Bhagavad Gita Chapter 10

VERSE 8

aham sarvasya prabhavo mattah sarvam pravartate
iti matva bhajante mam budha bhava-samanvitah

I am the source of everything. All things emanate from Me. Understanding this, the wise who are endowed with love, worship Me with all their heart.

VERSE 9

mac-chitta mad gata-prana bodhayantah parasparam
kathayantash cha mam nityam tushyanti cha ramanti cha

Those who are always thinking of Me, who have dedicated their lives to Me, enlighten each other and feel great satisfaction and joy by always speaking about Me.

VERSE 10

tesham satata-yuktanam bhajatam priti-purvakam
dadami buddhi-yogam tam yena mam upayanti te

To those who are constantly devoted to Me and worship Me with love, I continue to bestow upon them devotional inspiration by which they may come to Me.

VERSE 11

tesham evanukampartham aham ajnana-jam tamah
nashayamy-atma-bhavastho jnana-dipena bhasvata

Out of compassion for them, I appear in their hearts and destroy with the shining lamp of knowledge the darkness that is born from ignorance.

Anuvritti
Verses eight to eleven of this chapter are said to contain the ontological substance of Bhagavad-gita. Herein, Sri Krishna not only establishes Himself as the source of the material and spiritual worlds, and the origin of Brahman and Paramatma, but by the use of the word sarvasya, meaning all or everything, He also establishes Himself as the origin of Narayana in Vaikuntha (the Master of all potencies).
In the pantheon of demigods we find that Brahma is master of Satyloka, Shiva is the master of Shivaloka, Indra the master of Indraloka, but nowhere do we find it said that anyone is master and origin of everyone and everything except Sri Krishna. The understanding that Krishna is sarvasya compels the wise to worship Krishna in love with all their hearts.
Krishna says, mac-chitta – always think of Me, always meditate on Me; mad-gata-prana – dedicate your life to Me. In association with the wise engage in kathayanta, discussions about Me, enlivening and enlightening each other (bodhayanta). This is the proper way of meditation and engagement for the mind and senses.
Discussing about Krishna is called krishna-katha, or shravana and kirtana – hearing and chanting. This means to hear and chant Bhagavad-gita, Srimad Bhagavatam and other such literature and to perform sankirtana, the congregational chanting of the maha-mantra:

hare krishna hare krishna krishna krishna hare hare
hare rama hare rama rama rama hare hare

This hearing and chanting of the name and activities of Krishna constitutes the topmost system of yoga. Krishna verifies in the Padma Purana that He resides wherever His name is chanted:

naham tishthami vaikunthe yoginam hridayeshu va
tat tat tishthami narada yatra gayatnti mad bhaktah

I am not in Vaikuntha, nor in the hearts of the lesser yogis. I remain where My devotees engage in glorifying My activities and chanting My name. (Padma Purana)

Altogether, there are nine processes in bhakti-yoga to fully engage oneself in Krishna. Of these, hearing and chanting are the first and foremost:

shravanam kirtanam vishnoh smaranam pada-sevanam
archanam vandanam dasyam sakhyam atma-nivedanam
iti pumsarpita vishnau bhaktiti chen nava-lakshana

Hearing and chanting the maha-mantra and about the activities of Vishnu or Krishna, remembering Him, serving His lotus feet, offering Him respectful worship, offering prayers to Him, becoming His servant, considering Him as one’s dearest friend, and surrendering everything unto Him – these nine processes are accepted as bhakti-yoga, pure devotional engagement. (Srimad Bhagavatam 7.5.23)

In verse ten we find the word priti-purvakam. Priti means love, but this love should not be confused with the love and affections of this material world which are shared between conditioned living beings. Priti is the state of pure affection wherein no tinge or expression of selfishness or mundane lust can be found. Such love is the requirement for worshiping Krishna and such love is the key to an eternal bond with Krishna. Those who have attained such love for Krishna by bhakti-yoga are continuously empowered by Him with the inspiration of pure devotion (buddhi-yoga), by which they may come to Him.
Krishna dwells in the hearts of all living beings as the Paramatma, the Super Consciousness, and directs the wanderings of everyone. When one desires to know Krishna, to serve Krishna and to enter into an eternal relationship with Him, then out of compassion for them, Krishna personally manifests in their hearts and destroys with the shining lamp of knowledge the darkness born of ignorance. It is said that Krishna is light and ignorance is darkness:

krishna–surya-sama maya haya andhakara
yahan krishna tahan nahi mayara adhikara

Krishna is like the sun and maya (ignorance) is like the darkness. Wherever there is Krishna there can never be any darkness. (Chaitanya-charitamrita, Madhya-lila 22.31)

Wherever light appears darkness is vanquished. Thus, when Krishna personally appears in the heart of the bhakti-yogi, all darkness and despair vanish and one is supremely enlightened. This is the quintessence of Bhagavad-gita.