A question many times arise, many hold this opinion regarding upanisads that upanisads talks about Impersonal Brahman, Shiva etc. But this view is not truth rather absurd beacuse the teachings of Upanisads should be inline with teachings of Bhagavad Gita, Vedanta Sutra, Purusha Suktam, mahabharata, Ramayana etc. we can't take just few context out of any upanisads or any scripture and establish any view according to that, a truth can only be established when it supported by other scriptures.

Like some upanisads talks about impersonal aspect of god for e.g.

Svetasvatara Upanisad (3.10): tato yad uttarataram tad arupam anamayam/ ya etad vidur amrtas te bhavanti athetare duhkham evapiyanti.

This impersonalist use this verse to show gos has no form (Arupam). But this if we see without seeing other verses of the same upanisads then we really conclude that god is impersonal. But no , beacuse the same upanisads says

Other verses in the Svetasvatara Upanisad (3.8-9) substantiate this as follows:

vedaham etam purusam mahantam aditya-varnam tamasah parastat
tam eva viditvati mrtyum eti nanyah pantha vidyate 'yanaya
yasmat param naparam asti kincid yasman naniyo no jyayo 'sti kincit
vrksa iva stabdho divi tisthaty ekastenedam purnam purusena sarvam

"I know that Supreme Personality of Godhead who is transcendental to all material conceptions of darkness. Only he who knows Him can transcend the bonds of birth and death. There is no way for liberation other than this knowledge of that Supreme Person.

"There is no truth superior to that Supreme Person, because He is the supermost. He is smaller than the smallest, and He is greater than the greatest. He is situated as a silent tree, and He illumines the transcendental sky, and as a tree spreads its roots, He spreads His extensive energies."

Now few things to understand here that the upanisads first mentioned him arupam ( no form) again in other verses declared he is Purusha (Person) . So what should we conclude, it seems apparant contradiction , but there is no contradiction beacuse Arupam used for that the Purusha has no form like us, he has no material form rather he has a transcendental form.

Another thing if we accept few verse of upanisads talks about impersonal accept, and we conclude that absolute truth is impersonal on that basis, then what will we answer to those verse which talks god is personal.

Like in BG Krishna saying

BG 14.27: And I am the basis of the impersonal Brahman, which is immortal, imperishable and eternal and is the constitutional position of ultimate happiness.

BG 9.11: Fools deride Me when I descend in the human form. They do not know My transcendental nature and My supreme dominion over all that be.

BG 7.24: Unintelligent men, who do not know Me perfectly, think that I, the Supreme Personality of Godhead, Kṛṣṇa, was impersonal before and have now assumed this personality. Due to their small knowledge, they do not know My higher nature, which is imperishable and supreme.

How can we deny the verses of Isha upanisads:

Iso 15: O my Lord, sustainer of all that lives, Your real face is covered by Your dazzling effulgence. Kindly remove that covering and exhibit Yourself to Your pure devotee.

Iso 16: O my Lord, O primeval philosopher, maintainer of the universe, O regulating principle, destination of the pure devotees, well-wisher of the progenitors of mankind, please remove the effulgence of Your transcendental rays so that I can see Your form of bliss. You are the eternal Supreme Personality of Godhead, like unto the sun, as am I.

So shastra should be read as a whole, not as a part, else we will jump in worng conclusion, beacuse if we conclude god is impersonal then we have to refute Bhagavad Gita and Upanisads other statements. But is this possible, that means our understanding is wrong.

Also it happens sometime that other gods name like Shiva occuring in upanisads like in svetasvatara upanisads 5.4 , but this Shiva is nothing but other Name of Vishnu beacuse

27) shivah: He who is eternally pure

600) shivah: Auspiciousness

The both names appears in the 27th name of Vishnu in Vishnu sahsranama.

Again to confirm this Krishna saying

BG 10.23: Of all the Rudras I am Lord Śiva, of the Yakṣas and Rākṣasas I am the Lord of wealth [Kuvera], of the Vasus I am fire [Agni], and of mountains I am Meru.

So this confirms That Vishnu is Shiva and not the other way around, thus Upanisad can't say Siva is supreme in these verse , because its simply not supported by Vedanta-sutra, Bhagavd Gita, Purusha Suktam and other verse of Upanisads.

Hence Shastra should and can only be understandable when we follow proper source and bonafide paramapara rather than trying to intrepret on own whim. Any verse from any shastra shoulb be confirmed by Prashtana traya (Vedanta-sutra, Upanisads and BG) . Bhagavad Gita is the basis and other should be inline with this. Else that interpretation has no value.