The five wise virgins, watchful and alert, had taken oil in the vessels of their heart. That oil, not part of their own nature, means the grace of the Spirit. They were enabled to enter with the Bridegroom into the heavenly bridal chamber.
The other five virgins, however, were content with their own nature. They neither watched nor busied themselves to receive the oil of gladness [Ps. 44:6] in their vessels, while they were yet in the flesh. Instead, they sank into sleep through carelessness, slackness, idleness, and ignorance, or even fancied righteousness. Hence, they were shut out of the bridal chamber of the Kingdom, unable to give satisfaction to the heavenly Bridegroom. Held fast by their tie to the world, and by some earthly affection, they did not give their whole love or passionate devotion to the heavenly Bridegroom; thus they were not provided with oil.
Souls that seek the sanctification of the Spirit, which is outside of nature, fasten all their affection upon the Lord. It is in the Lord that they walk, pray, and employ their thoughts, turning away from all else. For this cause, they are priviledged to receive the oil of heavenly grace. This enables them to succeed in coming through without falling, giving perfect satisfaction to the spiritual Bridegroom. Those souls, however, content with what belongs to their own nature, creep upon the earth in their thoughts. Not only do they employ their thoughts upon earth, but their minds have their whole existence upon earth. In their own estimation, they appear to belong to the Bridegroom, and to be adorned with the ordinances of the flesh. Nonetheless, they have not been born of the Spirit; and they have not received the oil of gladness.
(Fifty Spiritual Homilies, Homily IV)