In all the Hindu rituals that I have attended, I have seen that there is always a fire burning. Why is fire so important?
Significance of fire in Rituals -
Agni is an ancient Hindu deity. Agni is one of the important Vedic Deity. Agni is the god of fire.
Agni as High-priest, messenger of gods -
Agni accepts sacrifices made to him from men who are devoted to supreme Lord. Agni is the supreme director of religious ceremonies and duties, serving as a high-priest, the messenger of gods, carries oblations directly to the gods from human beings. So we cannot reach any of the gods without Agni or fire as the medium. It is believed that Agni consumes ritualstic offerings, and transfer them to the respected gods through smoke.
अग्निमीळे पुरोहितं यज्ञस्य देवं रत्वीजम | होतारं रत्नधातमम ||
अग्निः पूर्वेभिर्र्षिभिरीड्यो नूतनैरुत | स देवानेह वक्षति ||
I Laud Agni, the chosen Priest, God, minister of sacrifice, the Hotar, lavishest of wealth. Rig-Veda Book 1 Hymn1
यो मर्त्येष्व अम्र्त रतावा देवो देवेष्व अरतिर निधायि |
होता यजिष्ठो मह्ना शुचध्यै हव्यैर अग्निर मनुष ईरयध्यै ||
The, Faithful One, Immortal among mortals, a God among the Gods, appointed envoy, Priest, best at worship, must shine forth in glory. Agni shall be raised high with man's oblations. Rig-Veda Book-4 Hymn 1
Agni as witness -
Agni is the only element of the five elements (Prithvi, Akash, Jal, Vayu and Agni) which cannot get altered or get polluted by the thing it purifies i.e., we can pollute any of the other five elements by the act of purifying with them but fire is not subject to this same constraint(for example, Water gets polluted once it is used to cleanse one's hands). Since it is the only element which does not get polluted, in any Hindu Marriage ceremony, Young couples are required to take a vow in front of Agni as witness since a Hindu Wedding is considered fully consecrated only when witnessed by fire (Agnisakshi ,अग्निसाक्षी)(for reasons described above). In this vow, Saptapadi or Saat Phere - which involves taking seven steps walking around the fire for completion of marriage, is done.
We also see Valmiki Ramayana describes how Lord Rama asked his wife Sita to affirm her chastity in the presence of fire.
अब्रवीत्तु तदा रामं साक्षी लोकस्य पावकः |
एषा ते राम वैदेही पापमस्यां न विद्यते || ६-११८-५
Then, the fire-god, the witness of the whole world, spoke to Rama as follows "Here is your Seetha. No sin exists in her."
Agni as a purifier -
The worship of Agni enables us to burn up our impurities, during performing ritual or in our day-to-day lives. When such work is done regularly with the flame of ritual Fire, many benefits are bestowed on us, to progress on the Path, e.g. Agnihotra. In Hinduism **preparing our daily food is also considered as a ritual or Yagna. **So Agni plays very vital role in cooking and also in purifying our food. So here, Agni shines as the most important element.
विश्वा अग्ने.अप दहारातीर्येभिस्तपोभिरदहो जरूथम | पर निस्वरं चातयस्वामीवाम || ७॥
ॐ अग्नॆ नय सुपथा रायॆ अस्मान् विश्वानि दॆव वयुनानि विद्वान् ।
युयॊध्यस्मज्जुहुराणमॆनो भूयिष्ठं तॆ नम उक्तिं विधॆम ॥ १८ ॥
AGNE NAYA SUPADHÂRÂYE ASMÂN VISWÂNI DEVA VAYUNÂNI VIDWÂN YUYODHYA ASMAD JUHURÂNÂM ENO BHÛISTÂN TE NAMA UKTIM VIDHEMA
Oh, Lord Agni, the foremost one,the knower of the intricate Paths of creation! Lead us by the simple righteous Path to felicity. Destroy our past Karma, eliminate from us the attraction to dubious and ignorant paths. We offer our worship to you. Ishavasya upanishad
Agni as a destroyer -
In Hindu tradition, bodies of dead persons are cremated, that is, burned through Agni - this is called Antyesti. Antyesti literally means "last sacrifice", and refers to the funeral rites for the dead in Hinduism. This rite of passage is one of traditional Saṃskāras in the life of a Hindu. It is also referred to as Antima Sanskar, Antya-kriya, Anvarohanyya, or as Vahni Sanskara**. So from this, we can see that Agni is very important even in last rites ritual done in Hinduism. Here
Fire is the Vedic Deity Agni.
In fire rituals(Yajnas), he(Agni) acts as the messenger & hence indispensable .
He carries the Havi(the oblations like Ghee,etc) to the receiver of the Yajna.
That is,Agni forms the bridge between the performer of the Yajna and the receiver of the oblations or the Gods.
So,understandably without Agni no Yajna is possible.
The following are a few Vedic verses that mention Agni as the messenger in fire sacrifices.
1)Agnim dutam Vrnimahe Hotaram Viswavedasam Yasya Yajnasya Sukratum
We choose Agni as our messenger,the herald,master of all wealth,well skilled in our sacrifice(Rig Veda 1.12.1,Taittirya Samhita 184.108.40.206a)
2)Pravas Sukraya Bhanave Bharadhvam ,Havyam Matim Cagneye suputam| Yo daivyani Manusa Janugmsi ,Antar Viswani Vidma na Jigati||
Bring forth your offerings to his refulgent splendour; your hymn as purest offering to Agni the mystic fire of wisdom who goes as messenger conveying all songs of men to the gods in heaven. (Rig Veda 7.4.1.)
3)For Yama extract the somajuice, to Yama offer the oblation into the sacred fire; to Yama goes this sacrifice well-prepared with Agni as it's messenger.(Yama Suktam)