The parable about the ten virgins has been clearly and convincingly explained by St. Seraphim of Sarov in his conversation with Motovilov:
Some say that the shortage of oil of the foolish virgins signifies their shortage of good works in their lives. Such understanding is not exactly correct. How can they be short of good works if they, though foolish, are still called virgins? Chastity is a supreme virtue, the state of being equal to angels, and could itself serve as a substitute for all other virtues. I humbly think that they were actually short of the grace of God's All-Holy Spirit. These virgins did good, and out of their spiritual foolishness supposed that doing good was exactly the point of Christianity. They did good works and by this obeyed God, but they did not care in the least beforehand whether they had received or reached the grace of God's Spirit. This very gaining of the Holy Spirit is that oil which the foolish virgins lacked. They were called foolish because they forgot about the necessary fruit of virtue, the grace of the Holy Spirit, without which no one is saved and no one can be saved, for: ‘it is by the Holy Spirit that any soul is vitalized and exalted in chastity, and any soul is lit by the Trinitarian unity in holy mysteries’. The Holy Spirit moves into our souls, and this installation of the All-Mighty into our souls, and co-existence of His Trinitarian Unity with our spirit is given only through the gaining by all means, the Holy Spirit, which prepares in our soul and body the throne for God’s creative co-existence with our spirit in strict accordance with the word of God: ‘I will dwell among them and will be their God, and they will be my people’. This is the oil in the lamps of wise virgins, oil that burnt bright and long, so that the virgins with the burning lamps could wait until the Bridegroom who came at midnight, and enter with Him into the house of joy. But the foolish virgins, seeing that their lamps were going out, went to the marketplace to buy oil but would not come back in time, for the doors were already shut. The marketplace is our life; the door of the house of marriage (that was shut and did not lead to the Bridegroom) is our human death; wise and foolish virgins are Christian souls; the oil is not works but the grace of the All-Holy Spirit of God which is received through these works, and which converts things perishable into things imperishable, transforms spiritual death into spiritual life, darkness into light, the manger of our being, with passions tied like cattle and beasts, into the Divine Temple, into the glorious palace of never-ending rejoicing in Christ Jesus.