4:38  

न हि ज्ञानेन सदृशं पवित्रमिह विद्यते ।
तत्स्वयं योगसंसिद्धः कालेनात्मनि विन्दति ॥४-३८॥

na hi dnyaanena sadRRishaM pawitramiha widyate ।

tatswayaM yogasaMsiddhaH kaalenaatmani windati ॥४-३८॥

Source: Automatic ITRANS Transliteration

Nothing as knowledge so holy here exists.
Truly, automatically, [one] firmly resolved in yoga in time attains the self.

Nothing so holy as knowledge exists here. Truly, [one] firmly resolved in yoga, in time, attains the self.

(See also 5:6.)

Source: Milind S. Pandit

Verily there is no purifier in this world like knowledge. One who becomes purified by Karma-yoga discovers this knowledge within (naturally) in course of time. (See also 4.31, and 5.06).

Source: The Bhagavad-Gita: The Sacred Song

In this world, there is nothing so sublime and pure as transcendental knowledge. Such knowledge is the mature fruit of all mysticism. And one who has achieved this enjoys the self within himself in due course of time.

Source: Bhagavad Gita As It Is