Cas' mother was a Kainian (fire Mage from Kain) which left her with dormant fire powers. When she was almost burned to death, her fire powers became active. She's not at the level of a Master yet, but she learns fast.
Luke John answered them all, “I baptize you with[a] water. But one who is more powerful than I will come, the straps of whose sandals I am not worthy to untie. He will baptize you with the Holy Spirit and fire.