Often we have the idea that the Upanisads support the impersonal conception of the Absolute Truth, but this is actually due to the many Mayavadi interpretations of the texts. In reality, the Upanisads directly speak about the personal form of the Lord, just like the Bhagavad-Gita or the Srimad Bhagavatam.

The difficulty is that the Upanisads are parts of the original Vedas, and thus the language is more difficult, and the concepts more abstract. Different from books like the Mahabharata or Ramayana, the Upanisads were written to be studied in the company of the spiritual master, receiving explanations, asking questions, and so on. Just being a Sanskrit scholar is not sufficient.

Unfortunately, not many of us have the opportunity to hear directly from a self-realized soul, but there is another process that can also work. This is based on the suggestions of Jiva Goswami in the Sat Sandharbas. To understand the Upanisads, we should start y studying the Vedanta Sutras, which have the conclusions of the Upanisads. To understand the Vedanta Sutras we should study the Govinda Bhasya of Srila Baladeva Vidyabhusana, who explains everything nicely.

However, the Govinda Bhasya is not an easy book. To understand it we need to have a very good grasp of the philosophy, understanding the references, and the correct conclusions. To get that we need to study the books of Srila Prabhupada very carefully.

When we understand the conclusions, examples, and references Prabhupada gives in his books, especially the Srimad Bhagavatam, we can understand well the Govinda Bhasya, and from there we may venture into trying to understand the Upanisads.

In any case, this is a passage from the Taittiriya Upanisad I was working on. This is one example of how the Upanisads speak clearly about a personal God, who is the controller of multiple potencies and the source of ultimate bliss:

“He who sees Brahman as non-existing becomes degraded. He who knows the Brahman as existing, flourishes and becomes a saint. The embodied Self of this bliss is the same as that of the former, the vijñānamaya (He has a human form).
Following, the student asked: Does the one who doesn’t know (the Lord) ever go to that world of bliss (the spiritual world) after departing from this life? Or only the one who knows goes there?”
The answer was: “The Lord desired, ‘May I become many; may I increase.’ He then contemplated Himself. After He had thus contemplated, He created all beings and everything that exists. He then entered that creation, and having entered it He became both manifested and unmanifested, defined and undefined, supported and unsupported, endowed with consciousness and not conscious, real and unreal. Brahman became all of this (He became everything that exists), and thus the wise call Him Satya, the absolute truth.
In the beginning, the creation was non-existent. It was not yet defined by forms and names. From this unmanifest, was born whatever exists. The Lord made it, therefore He is called the Self-born. Who could breathe and go on living if the Lord was not in his heart? He alone brings auspiciousness. When one takes shelter in Him, he attains fearlessness. For one who deviates from it, however, there is fear. This fear exists only for the one who thinks himself wise, and not for the true sage.
From fear of Him, the wind blows. From fear of Him, the sun rises. From fear of Him, Agni, Indra and even Yamaraja perform their duties.
Now, hear about the definition of ānanda, bliss:
Take a noble young man, who is well versed in the Vedas, very intelligent, handsome, strong. Let the world be full of wealth for him. That is the measure of human bliss.
One hundred times that human bliss is one measure of the bliss of the lower Gandharvas. One hundred times more is the bliss of the divine Gandharvas (in Swargaloka), and one hundred times more is the bliss of the inhabitants of Pitrloka. One hundred times more is the bliss of the demigods, one hundred times more is the bliss of the sacrificial devas, and one hundred times more is the bliss of the 33 principal demigods. One hundred times more than this is the bliss of Indra, and one hundred times more is the bliss of Brhaspati. One hundred times more still is the bliss of the Prajapati (Brahma), and one hundred times more is the bliss of Brahman, and likewise of a great sage who is free from desires.
He who knows this Brahman in man (as Paramatma) and in the sun (as Surya-Narayana) and sees both as one, reaches this Supreme Brahman, the anandamaya, who is also the source of the annamaya, prāṇamaya, manomaya, and vijñānamaya.
He who knows the bliss of that Brahman, Whom the mind fails to find, fears nothing. Material thoughts don’t affect him. He doesn’t rejoice with the good or lament for the evil. He finds equanimity in both. Indeed, this is the Upaniṣad.”