Leviticus 6:12
And the fire upon the altar shall be burning in it; it shall not be put out: and the priest shall burn wood on it every morning, and lay the burnt offering in order upon it; and he shall burn thereon the fat of the peace offerings. And the fire upon the altar shall be burning in it; it shall not be put out: and the priest shall burn wood on it every morning, and lay the burnt offering in order upon it; and he shall burn thereon the fat of the peace offerings.
The fire on the altar was kindled by God Himself and was to be kept burning continually, without interruption and never to be put out. The priest was to tend it every morning by burning wood on it, laying the burnt offering in order upon it, and burning thereon the fat of the peace offerings. This same fire was to be used for all sacrifices and for the sanctuary service. No common fire was to be used in sacred service or substituted for this God-kindled fire.
The sacred fire represented God Himself. It represented the divinely established communication and connection between God and humanity opened through Jesus Christ. Incense kindled from this fire was placed in censers and offered so that its fragrance rose before God with the prayers of the people waiting on Him, representing Christ's mediation.
The verse requires worship to follow God's instructions exactly with strict obedience, preserving the distinction between what is sacred and what is common. Using unauthorized, common fire violates His express directions and dishonors His holiness. Nadab and Abihu violated this command by using common fire that God had not commanded.
The continually maintained fire for God's people is like the living water Christ gives, which continually meets spiritual need and springs up to everlasting life. Spiritually, only at God's altar can people kindle their lights with divine fire. That light reveals human weakness and Christ's perfection and purity. Beholding Jesus awakens the desire to be like Him, seeing His righteousness creates hunger for it, and earnest prayer brings the heart's desire to God.