The jurisdiction of Krishna consciousness extends everywhere, and one who knows Krishna consciousness is blessed. Those who do not know Krishna are in illusion, and so knowledge of Krishna is liberation, and ignorance of Him is bondage.
His Divine Grace A. C. Bhaktivedanta Swami Prabhupada
Bhagavad-gita As It Is 7.8, Purport

Just because mundane thoughts appear in the mind while we chant the holy name does not mean we should slacken our chanting. The useless thoughts will gradually disappear as the irrevocable fruits of chanting the holy names appear. Do not be in a hurry.
Srila Bhaktisiddhanta Sarasvati Thakura
Amrita-vani, Section: “108 Essential Instructions,” No. 93

Moral codes that do not include thought of the Lord cannot accomplish any good for humanity even though they may be presented most skillfully, because such morality is purely materialistic. Only moral codes that express belief in the Lord and oblige service to Him are fit for mankind.
Srila Bhaktivinoda Thakura
Sri Chaitanya-shikshamrita, First Rainfall, First Shower

A heart filled with love for the Lord can naturally understand the truth that the Lord’s form is identical with the Lord, just as an ear trained to understand music can naturally understand the forms of various melodies.
Srila Baladeva Vidyabhushana
Commentary on Vedantra-sutra 3.2.14

O my Lord, those influenced by demoniac principles cannot realize You, although You are clearly the Supreme by dint of Your exalted activities, forms, character, and uncommon power, which are confirmed by all the revealed scriptures in the quality of goodness and the celebrated transcendentalists in the divine nature.
Srila Yamunacharya
Stotra-ratna 12

The Lord is pleased only if one has unflinching, unalloyed devotion to Him. Without sincere devotional service, everything is simply a show.
Sri Prahlada Maharaja
Srimad-Bhagavatam 7.7.52

For those who have achieved the Supreme Personality of Godhead, the master of the pure devotees, what is impossible to do, and what is impossible to give up?
Sri Durvasa Muni
Srimad-Bhagavatam 9.5.15

Krishna alone, the controller of all, pervades everything, and He is the true object of worship. Although one, He appears as many. The wise who constantly worship Him within their hearts enjoy eternal happiness, a happiness no one else can enjoy.
Lord Brahma
Gopala-tapani Upanishad, Purva 1.21

If someone says, “The individual soul’s becoming the Supreme is the true goal of life,” then that person indeed speaks a lie. The individual soul can never become the Supreme.
Vishnu Purana 2.14.27