Holy souls, you cannot move ahead unless the fire has stung you first: enter the flames,
Fire is a symbol that can be used in a variety of ways but, I argue that since we are on the terrace of the lustful and at the threshold of Earthy Paradise that the fire in this particular instance is used to portray the notion that everybody has been burned in love. Recall that while Dante is not literally burned as he walks through the flames. The flames are however, intensely hot to the point where he would rather have thrown himself into molten glass to find coolness. My point being love creates within us a swell of emotions and passion that "heat up" so to say; generally speaking everyone has felt this. But, when the object of our burning desire is not as passionate as we are, when they act cold, cruel and harsh toward us it hurts. We are not literally burned but figuratively burned. Dante must experience this feeling of love, desire and pain before he is able to reach Beatrice for obvious reasons. And in this way the fire also serves as a liminal space.