Verse 4.17.10

मानवो ब्रह्मैवैक ऋत्विक्कुरूनश्वाभिरक्षत्येवंविद्ध वै ब्रह्मा यज्ञं यजमानं सर्वांश्चर्त्विजोऽभिरक्षति तस्मादेवंविदमेव ब्रह्माणं कुर्वीत नानेवंविदं नानेवंविदम् ॥ ४.१७.१० ॥
॥ इति चतुर्थोऽध्यायः ॥

mānavo brahmaivaika ṛtvikkurūnaśvābhirakṣatyevaṃviddha vai brahmā yajñaṃ yajamānaṃ sarvāṃścartvijo'bhirakṣati tasmādevaṃvidameva brahmāṇaṃ kurvīta nānevaṃvidaṃ nānevaṃvidam || 4.17.10 ||
|| iti caturtho'dhyāyaḥ ||

10. A good brahmā priest is one who is able to observe silence, or one who is thoughtful. Just as a horse protects the soldiers, a learned brahmā priest protects the sacrifice, the sacrifices, and all the other priests. Therefore, one should appoint only such a learned brahmā for one’s sacrifice. One should not appoint anyone else.

Word-for-word explanation:

Mānavaḥ brahmā eva ekaḥ ṛtvik, a true brahmā priest is one who is thoughtful [or, is able to observe silence]; kurūn aśvā abhirakṣati, [just as] a horse protects the soldiers; evamvit ha vai brahmā, such a learned brahmā priest; yajñam yajamānam sarvān ca ṛtvijaḥ abhirakṣati, protects the sacrifice, the sacrificer, and all the other priests; tasmāt, therefore; evam vidam eva brahmāṇam kurvīta, one should appoint only such a learned brahmā priest; na anevam vidam, not one who is otherwise; na anevam vidam, not one who is otherwise. Iti saptadaśaḥ khaṇḍaḥ, here ends the seventeenth section. Iti chāndogyopaniṣadi caturthaḥ adhyāyaḥ, here ends the fourth chapter of the Chāndogya Upaniṣad.

Commentary:

When a priest is able to maintain silence and is by nature thoughtful, he may be appointed as the brahmā for a sacrifice. He is the chief priest by virtue of his scholarship and his character. The Upaniṣad compares him to a horse which protects his master when he is in trouble. A good brahmā priest can take care of the sacrifice, the sacrifices, and the subordinate priests. For this reason, only one who is qualified should be appointed. One who is not fit should not be given the honour.