Isaiah 6:6

Isaiah only acknowledged his hopeless condition-he did not plead with God or make vows to God-and God then went into action. Confession must precede cleansing (cf. 1 John 1:9). The altar from which the seraph took the coal was probably the brazen altar in heaven, in which case the coal itself symbolizes substitute sacrifice. [Note: Motyer, p. 78.] Fire from the brazen altar lit the incense on the incense altar in Israel, so, whichever altar may be in view, the coal connects with sacrifice. Fire in the Old Testament symbolizes the wrath of God (Genesis 3:24; Numbers 11:1-3), the holiness of God (Exodus 3:2-6; Exodus 19:18-25), His purifying process (Numbers 31:22-23; Malachi 3:2-3), and the context of the Law (Deuteronomy 4:12; Deuteronomy 4:33; Deuteronomy 4:36).

"A seraph peels off from his flight path around the throne, diving straight for Isaiah. He’s holding a burning coal that he took from the altar with tongs, but not because it is hot. After all, a seraph himself is a burning one. He took this coal with tongs because it is a holy thing. It belongs to the place of sacrifice and atonement and forgiveness. But this holy thing touches Isaiah’s dirty mouth, and it does not hurt him, it heals him. What we must see, in the context of the whole Bible, is that this burning coal symbolizes the finished work of Christ on the cross." [Note: Ortlund, pp. 79-80.]