Verse 8.5.1

अथ यद्यज्ञ इत्याचक्षते ब्रह्मचर्यमेव तद्ब्रह्मचर्येण ह्येव यो ज्ञाता तं विन्दतेऽथ यदिष्टमित्याचक्षते ब्रह्मचर्यमेव तद्ब्रह्मचर्येण ह्येवेष्ट्वात्मानमनुविन्दते ॥ ८.५.१ ॥

atha yadyajña ityācakṣate brahmacaryameva tadbrahmacaryeṇa hyeva yo jñātā taṃ vindate'tha yadiṣṭamityācakṣate brahmacaryameva tadbrahmacaryeṇa hyeveṣṭvātmānamanuvindate || 8.5.1 ||

1. Then that which is known as yajña [sacrifice] is brahmacarya. This is because one who knows the Self attains Brahmaloka through brahmacarya. Again, that which is known as iṣṭa [worship] is brahmacarya, for the desired Self is attained through brahmacarya.

Word-for-word explanation:

Atha, then; yat, that which; yajñaḥ iti ācakṣate, is called ‘yajña’ [sacrifice]; tat brahmacaryam eva, that is brahmacarya; hi, because; yaḥ jñātā, one who knows this; brahmacaryeṇa, through brahmacarya; tam vindate, attains that [Brahmaloka]; atha, then; yat iṣṭam iti ācakṣate, that which is called ‘iṣṭa’ [worship]; tat brahmacaryam eva, that is brahmacarya; hi, because; brahmacaryeṇa eva, through brahmacarya; iṣṭvā ātmānam anuvindate, one attains the desired Self.

Commentary:

Whatever you do with the idea of self-restraint is brahmacarya. Worship is brahmacarya; fasting is brahmacarya; observing silence is brahmacarya. If you retire from the world and go to the forest, that also is brahmacarya. The whole idea is self-restraint. Many people observe silence for a certain length of time. Gandhiji, for instance, would observe absolute silence once a week. Some people may say this is silly, but it’s not. Such observances bring strength.

Suppose something has provoked you and you are angry, but through strength of mind you get control of your emotions and you do not retaliate. Similarly, you may feel tempted to do something you shouldn’t and you refuse to do it. Or you may see some good food and feel tempted to eat something that is bad for you, but then you restrain yourself. Self-control in any form is brahmacarya. It is brahmacarya because it ultimately leads to Self-realization and union with Brahman.

The Upaniṣad says here that the performance of a sacrifice is also brahmacarya because there are many restrictions imposed on the sacrificer. For instance, he cannot eat until he finishes the worship. The word for sacrifice is yajña, which comes from yaḥ

Another reason why worship is called brahmacarya is that the worship is done to a particular deity (iṣṭa), and you must meditate on that deity. That is, you are wishing (iṣ) for that deity, or wishing for your union with that deity.

The only way to control the mind is through mental strength. According to Vedānta, strength is not outside. It is within. And self-control is the source of strength. First and foremost is brahmacarya. In fact, it is first and last. It’s not that you practise self-control for some time and then you say, ‘I don’t need to observe this any more.’ No, all through we need self-control. Gradually, however, it becomes natural. By constantly observing truthfulness, you find you are incapable of saying what is immoral or wrong.