From reformed perspective, what does the oil in the foolish virgin's lamp represent? This question is related to the question: How do "once saved always saved" advocates explain the parable of the wise virgins?

Many of the answers explain that the foolish virgins never had oil, however one answer pointed out that the difference is they did not take extra oil in a vessel. "But the wise took oil in their vessels with their lamps" (Mathew 25:4)

It does appear their lamps were burning since both "trimmed their lamps" (Matt 25:7) and the foolish said to the wise "our lamps are gone out" (Matt 25:8). 

If this is the case, and the foolish virgins had oil in the lamp but it ran out, what does the oil signify?
John Calvin in his commentary on Matthew encourages us to not get bogged down in the minutia of the parables. However, he essentially equates the oil to zeal.


  Some people give themselves a good deal of uneasiness about the lamps, the vessels, and the oil; but the plain and natural meaning of the whole is, that it is not enough to have ardent zeal for a short time, if we have not also a constancy that never tires. And Christ employs a very appropriate parable to express this. A little before, he had exhorted the disciples, that as they had a journey to perform through dark and dreary places, they should provide themselves with lamps; but as the wick of the lamp, if it be not supplied with oil, gradually dries up, and loses its brightness, Christ now says, that believers need to have incessant supplies of courage, to support the flame which is kindled in their hearts, otherwise their zeal will fail ere they have completed the journey.