There is no difference between the kingdom of God and the devotional service of the Lord. Since both of them are on the absolute plane, to be engaged in the transcendental loving service of the Lord is to have attained the spiritual kingdom.
His Divine Grace A. C. Bhaktivedanta Swami Prabhupada
Bhagavad-gita As It Is 2.72, Purport

One who is not connected with the Supreme [in Krishna consciousness] can have neither transcendental intelligence nor a steady mind, without which there is no possibility of peace. And how can there be any happiness without peace?
Lord Sri Krishna
Bhagavad-gita 2.66

Lord Krishna, the one master and the Lord of the universe, is worthy of being served by everyone. Indeed, everyone is but a servant of His servants. That same Lord Krishna has descended as Lord Chaitanya, the Supreme Personality of Godhead. Everyone, therefore, is His servant.
Srila Krishnadasa Kaviraja Goswami
Sri Chaitanya-charitamrita, Adi 6.83–84

Wherever women are respected, even the demigods like to reside there, and wherever women are not respected, all actions are fruitless.
Manu-samhita 3.56

The study of the Vedas and the Puranas—verily, the study of all scriptures—is completed in all respects by one who studies the Gita with devotion.
Srimad Bhagavad-gita-mahatmyam
From Sri Vaishnaviya-tantra-sara

O my Lord, all the great sages headed by Srila Vyasadeva are most experienced regarding the transcendental truth. This is because they discuss and deliberate on the revealed scriptures again and again. Thereby they manifest these sacred books, which reveal Your eternal personal form abounding in all virtuous qualities, and Your divine activities, which are the essence of ambrosial nectar.
Srila Bhaktivinoda Thakura
Sri Gita-mala 1.3.1

It is the first duty of the most compassionate souls to divert the wretched taste of the living entities. It is more beneficial to save even one soul from Mahamaya’s prison house than to open millions of hospitals.
Srila Bhaktisiddhanta Sarasvati Thakura
Amrita Vani, 108 Instructions (#21)

I worship Govinda, the primeval Lord, who is Syamasundara, Krishna Himself, with inconceivable innumerable attributes, whom the pure devotees see in their heart of hearts with the eye of devotion tinged with the salve of love.
Lord Brahma
Sri Brahma-samhita 5.38