Bg 4.39

sraddhaval labhate jnanam
tat-parah samyatendriyah
jnanam labdhva param santim
acirenadhigacchati

Word for word: 
sraddha-van — a faithful man; labhate — achieves; jnanam — knowledge; tat-parah — very much attached to it; samyata — controlled; indriyah — senses; jnanam — knowledge; labdhva — having achieved; param — transcendental; santim — peace; acirena — very soon; adhigacchati — attains.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
A faithful man who is dedicated to transcendental knowledge and who subdues his senses is eligible to achieve such knowledge, and having achieved it he quickly attains the supreme spiritual peace.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Such knowledge in Krishna consciousness can be achieved by a faithful person who believes firmly in Krishna. One is called a faithful man who thinks that simply by acting in Krishna consciousness he can attain the highest perfection. This faith is attained by the discharge of devotional service and by chanting Hare Krishna, Hare Krishna, Krishna Krishna, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare, which cleanses one’s heart of all material dirt. Over and above this, one should control the senses. A person who is faithful to Krishna and who controls the senses can easily attain perfection in the knowledge of Krishna consciousness without delay.