First of all, why are you concerned about what trinitarians are telling you? If they don't understand who Jesus is, why should we think they would understand about baptism? To answer your question, when John the Baptist said that Jesus would baptize with the Holy Spirit and fire, the fire was intended to describe the effects of the Spirit on a person. First, like fire, the Spirit would remove the coldness a person's heart and fire him up with passion for God. Second, like fire, it would burn away all the impurities in aAnd tife. Third, like fire, it would provide illumination so a person could understand the ways of the Spirit.