Bg 17.26-27

sad-bhāve sādhu-bhāve ca
sad ity etat prayujyate
praśaste karmaṇi tathā
sac-chabdaḥ pārtha yujyate

yajñe tapasi dāne ca
sthitiḥ sad iti cocyate
karma caiva tad-arthīyaṁ
sad ity evābhidhīyate

Synonyms: 

sat–bhāve — in the sense of the nature of the Supreme; sādhu–bhāve — in the sense of the nature of the devotee; ca— also; sat — the word sat; iti — thus; etat — this; prayujyate — is used; praśaste — in bona fide; karmaṇi — activities; tathā — also; sat–śabdaḥ — the sound sat; pārtha — O son of Pṛthā; yujyate — is used; yajñe — in sacrifice; tapasi — in penance; dāne — in charity; ca — also; sthitiḥ — the situation; sat — the Supreme; iti — thus; ca — and; ucyate — is pronounced; karma — work; ca — also; eva — certainly; tat — for that; arthīyam — meant; sat — the Supreme; iti — thus; eva — certainly; abhidhīyate — is indicated.

Translation: 

The Absolute Truth is the objective of devotional sacrifice, and it is indicated by the word sat. The performer of such sacrifice is also called sat, as are all works of sacrifice, penance and charity which, true to the absolute nature, are performed to please the Supreme Person, O son of Pṛthā.

sad-bhāve sādhu-bhāve ca, in sad-bhāve that means when one is referring to the Absolute Truth, sādhu-bhāve, sad is Absolute Truth Supreme, sadhu is the devotees sādhu-bhāve ca

sad ity etat prayujyate, the word sad is used

praśaste karmaṇi tathā, when one is doing different kinds of work one is doing this, and

sac-chabdaḥ pārtha yujyate, and the sound sat is used

So Krishna has first talked about OM then He has talked about TAT and now He is talking about SAT.

yajñe tapasi dāne ca
sthitiḥ sad iti cocyate, that satis used ucyate, pronounced while doing yajña tapasa and dāna,

karma caiva tad-arthīyaṁ, karma caiva and while doing karma for what? sad ity evābhidhīyate, while doing Karma also sat is indicated through this.

Here praśaste karmaṇi refers to the prescribed duties, the prescribed duties we perform for various purposes, there is material sva-dharma and there is transcendental sva-dharma

Material sva-dharma is as per our Varna and Ashram we do our duties. And transcendental sva–dharma is for our practice of Bhakti and ultimately getting liberation from material existence.

So praśaste karmaṇi whatever material and spiritual duties one do that we can connect with Absolute Truth. Spiritual dharma is connected and material dharma also we can connect.

Starting question was about person who has faith but not in scripture so Krishna said that one has to come to the level where one is having faith in scripture. And chanting of OM TAT SAT can help one to come from non-scriptural to scriptural faith which is like second name of God. Chanting of other names of God will also help, those name should be referring to the attributes of God as per scriptures.

End of transcription.