The sastras repeatedly stress the incredible power of chanting Krishna's holy names. Harinama, even without diksa, or any preliminary rites of purification immediately produces results as soon as it contacts the tongue, "The Hare Krishna maha-mantra is so amazingly powerful that it gives fruit as soon as it touches the tongue. This mantra removes great sins that cannot be destroyed byyajna, tapa, dana, or other means. It is freely available and can be chanted by anyone, anywhere, at any time. Unlike other mantras, the chanting of the Hare Krishna maha-mantra does not depend upon diksa, proper behavior, or rituals of purification." (Padyavali 29).
Despite the astonishing potency of harinama, according to Sri Jiva Gosvami, one must still follow mahajanas like Narada Muni who first took diksa and then worshiped their beloved Deities. Without accepting mantra diksa from a spiritual master one cannot awaken his relationship with the Supreme Personality of Godhead as a servant, friend and so on. Only Sri Guru can establish one's eternal relationship with Krishna.
Sri Caitanya Mahaprabhu, His followers, and all of the Gaudiya Vaisnavas down to the present day have honored this tradition. Besides chanting the Hare Krishna maha-mantra. Lord Caitanya, Lord Nityananda and Sri Advaita Acarya all took mantra diksa from a bona fide spiritual master. To remain on the path of vaidhi-sadhana one needs only the Hare Krishna maha-mantra.
In order to progress on the platform of raga-marga and eventually serve Krishna's eternal vraja-lilas one must receive diksa mantras such as the Kama-bija, Gopala-mantra and Kama-gayatri. The Hari-bhakti-vilasa explains that these mantras are absolutely necessary for internal worship through meditation and for external worship of the arca-vigraha of Radha-Krishna.
The Hare Krishna maha-mantra is the nama atmika, the very life of Gopala-mantra and Kama-gayatri. The holy name of Krishna is the original mantra vibrating in the spiritual world of Goloka Vrndavana. These mantras (Gopala-mantra, Kama-gayatri) and other Vedic mantras have life because different names of Krishna are within them. Sripada Madhvacarya explained that the Vedic mantras used in demigod worship have potency because they contain secondary names of Krishna (gauna-nama).
In reality, mantras like om namo sivaya and others which are used to petition various demigods are indirectly glorifying Krishna. The word sivaya for example means "all-auspicious." Indeed, everything about the Supreme Lord Krishna is completely auspicious. Besides that, it is Sri Krishna alone who invests the demigods with their marvelous powers.
Of all mantras, Krishna's holy name is the most important. Sri Jiva Gosvami says Gayatri-mantra i.e. Kama Gayatri has value because it contains Krishna's names within it. Being full and complete (nitya mukto, purna suddho), the holy name can give everything required. The Gayatri mantra, however, is accepted to improve one's worship of the holy name.
The material world is a perverted reflection of the pure spiritual realm. In the spiritual world the Hare Krishna maha-mantra is at the top, but it becomes reflected in the material world as the mantra Om. The mantra Om is seen throughout the Vedas, and the other Gayatri mantras are found in the Smrtis. Then later in subsequent Vedic literatures the Hare Krishna maha-mantra becomes manifest. Because of the maha-mantra's seeming obscurity, the smarta brahmanas do not consider chanting Hare Krishna as important as chanting the Vedas and the Gayatri mantras. In reality, however, chanting the Hare Krishna maha-mantra is the very essence of all the Vedas.
Excerpt from Gayatri Mahima Madhuri by Mahanidhi Swami