Here is an authority speaking about Śrī Kṛṣṇa as the original Personality of Godhead and the first Nārāyaṇa. Even such an impersonalist as Ācārya Śaṅkara has said in the beginning of his commentation on the Bhagavad-gītā that Nārāyaṇa, the Personality of Godhead, is beyond the material creation. The universe is one of the material creations, but Nārāyaṇa is transcendental to such material paraphernalia.
Bhīṣmadeva is one of the twelve mahājanas who know the principles of transcendental knowledge. His confirmation of Lord Śrī Kṛṣṇa's being the original Personality of Godhead is also corroborated by the impersonalist Śaṅkara. All other ācāryas have also confirmed this statement, and thus there is no chance of not accepting Lord Śrī Kṛṣṇa as the original Personality of Godhead. Bhīṣmadeva says that He is the first Nārāyaṇa. This is also confirmed by Brahmājī in the Bhāgavatam (10.14.14). Kṛṣṇa is the first Nārāyaṇa. In the spiritual world (Vaikuṇṭha) there are unlimited numbers of Nārāyaṇas, who are all the same Personality of Godhead and are considered to be the plenary expansions of the original Personality of Godhead, Śrī Kṛṣṇa. The first form of the Lord Śrī Kṛṣṇa first expands Himself as the form of Baladeva, and Baladeva expands in so many other forms, such as Saṅkarṣaṇa, Pradyumna, Aniruddha, Vāsudeva, Nārāyaṇa, Puruṣa, Rāma and Nṛsiṁha. All these expansions are one and the same viṣṇu-tattva, and Śrī Kṛṣṇa is the original source of all the plenary expansions. He is therefore the direct Personality of Godhead. He is the creator of the material world, and He is the predominating Deity known as Nārāyaṇa in all the Vaikuṇṭha planets. Therefore, His movements amongst human beings is another sort of bewilderment. The Lord therefore says in the Bhagavad-gītā that foolish persons consider Him to be one of the human beings without knowing the intricacies of His movements.
The bewilderment regarding Śrī Kṛṣṇa is due to the action of His twofold internal and external energies upon the third one, called marginal energy. The living entities are expansions of His marginal energy, and thus they are sometimes bewildered by the internal energy and sometimes by the external energy. By internal energetic bewilderment, Śrī Kṛṣṇa expands Himself into unlimited numbers of Nārāyaṇas and exchanges or accepts transcendental loving service from the living entities in the transcendental world. And by His external energetic expansions, He incarnates Himself in the material world amongst the men, animals or demigods to reestablish His forgotten relation with the living entities in different species of life. Great authorities like Bhīṣma, however, escape His bewilderment by the mercy of the Lord.