Verse 3.1.5

सत्येन लभ्यस्तपसा ह्येष आत्मा सम्यग्ज्ञानेन ब्रह्मचर्येण नित्यम् ।
अन्तःशरीरे ज्योतिर्मयो हि शुभ्रो यं पश्यन्ति यतयः क्षीणदोषाः ॥ ५ ॥

satyena labhyastapasā hyeṣa ātmā samyagjñānena brahmacaryeṇa nityam |
antaḥśarīre jyotirmayo hi śubhro yaṃ paśyanti yatayaḥ kṣīṇadoṣāḥ || 5 ||

5.This Atman within the body, resplendent and pure, can be reached by truth and tapas, by sound knowledge and by abstinence from sexual pleasures constantly practised; he is within the body, resplendent and pure; him, assiduous Sanyasins see, their faults removed.

Shankara’s Commentary:

Com.—Now truth and the rest chiefly characterized by restraint are enjoined upon a bhikshu, i.e., mendicant, as auxiliary aids with sound knowledge; he should be attained by truth, i.e., by abstaining from falsehood; as also by tapas, i.e., by concentration of the senses and the mind, which is declared to be the highest tapas; for, it is this which is of greatest help, as it is turned towards beholding the Atman, not the other forms of tapas, such as the performance of the chandrayana (a penance), etc. “This Atman should he attained” should be read into every clause. ‘By good knowledge’, by beholding the Atman as it really is. ‘By Brahmacharya,’ by abstinence from sexual pleasure. Nityam, always. The word “always” should be read with every one of the words ‘truth,’ ‘tapas,’ etc. Just as a lamp within a building illumines every part of it, it will be said, later on, that they seethe Atman in whom there is neither deceit, nor falsehood, nor cunning. Who this Atman is that should be attained by these aids is explained. ‘Within the body,’ in the midst of the body, i.e., in the akas of the lotus of the heart. ‘Resplendent’, of golden hue. Subhra, pure. The yatayah, i.e., those who are habitually seeking, i.e., the Sanyasins, ‘their faults removed,’ i.e., devoid of all taint of mind such as anger, etc., find this atman. The drift is that the atman is attained, by Sanyasins with these aids as truth, etc., constantly practised and cannot be attained by them occasionally practised. This text is a eulogy of such aids as truth, etc.