The Gospel of Matthew is a sobering book for any Christian to read (just ask my old congregation at FKPCW, who had to endure a year of my preaching through it at the end of my ministry there). The first thing one must realize when he begins to read it though is that it is not a book about justification. Let me clarify that statement. It has implications for justification. It has implications for what kind of faith justifies us. But it doesn't deal specifically with justification itself. This is important to point out because many people read Matthew, or the Sermon on the Mount, as saying that what Christ commands is so great for us to fulfill that we can't do it, thus showing us our need for faith in Christ. That is absolutely a correct application of this passage, if Christ were speaking about how we might be justified. If that was his subject, then, of course, we would never make it. But that isn't his point, and that's not the focus of Matthew. The point of the Gospel of Matthew is to tell us what kind of faith saves us, not that faith saves us. Do you see the difference? One is talking about the nature of the means by which we are saved and the other is talking about the means by which we are saved. We are saved by faith, as we know from the rest of Scripture, but Matthew (like James) is focusing in on an important teaching that was very much a large part of Christ's ministry that explains to people who think they are saved by identifying themselves as God's servants, that true faith produces a change in one's life in terms of what he does (i.e., that faith works).
So, on the one hand, the Sermon on the Mount deals with people who think they are saved, but live their lives in disobedience to the will of God as it is revealed in Scripture (either in disobedience to its explicit commands, but more emphasizing the rebellious nature manifest within their seeking to only technically obey what is explicitly commanded while ignoring the underlying principles and implications of what is said). Hence, the Sermon on the Mount deals with sins that we often call sins of commission (i.e., sins that consist of evil things that we do, or sins of action).
On the other hand, as Matthew starts to end his presentation of Christ's teaching, he emphasizes sins of omission (i.e., sins that we commit by not doing good, or sins of inaction). That's what we have in Matthew 25, which together with the Sermon on the Mount functions as an inclusio, sandwiching the entire teaching of Christ in the Gospel together, and conveying to us the nature of true faith as one that produces both an abstention from sins of commission on the one hand and the doing of good on the other.
Most evangelicals miss this because they are busy thinking about Matthew in terms of justification (i.e., how we are saved) versus sanctification (i.e., what occurs when one is truly saved). So let's look at Matthew 25 for a moment. At the end of the chapter, in vv. 32-46, we have what is called the "Judgment of the Nations" passage, a name that often gives off the wrong impression to modern day interpreters, as this is really the judgment of the Christians of the nations, the reference to the nations functioning as a way to communicate to the Jewish people that God's kingdom includes the Gentiles (which is something Christ has been saying throughout). The terminology of sheep and goats deals, not with believers and unbelievers, as some suppose, but of professed believers, some of which are genuinely so (i.e., the sheep) and others who are not (i.e., the goats). Both of these animals exist in the same pastures. They hang out together.
Christ goes on to say that the distinction between them is that the one group (i.e., the sheep) does good (seeks to preserve the lives of their fellow Christians, e.g., by giving food, clothing, shelter, comfort in times of distress and hopelessness, etc.). The goats, on the other hand, are not in the practice of doing good. They are about their own lives, not the lives of others (as we also see from the Sermon on the Mount in a different way since people their are seeking to self-direct their lives rather than to give them up to someone else, specifically to Christ). There is no sense that these people ever belonged to Christ. They never had genuine faith. They were never saved. They only though they were because of their profession. But their profession did not manifest itself in the doing of good to fellow Christians (and this is specific to what is done to Christians, i.e., the least of "these brothers of Mine").
So what does this all tell us about the Parable of the Virgins? It tells us that this parable, as all of the parables, is about distinguishing true believers from false ones by looking at their fruits, specifically whether they do good to other Christians. Again, good isn't being nice. Don't confuse our modern concepts of superficial gestures that exist for ourselves with true good which is sacrificial and makes us bleed to do it. This is real good, the kind that truly seeks to preserve the lives of one's fellow Christians in tangible terms.
This is the context of the parable, and this then tells us what the oil that some have enough of, and others lack, represents. The oil is not the Holy Spirit, as some have suggested. That is not Matthew's point. It's absolutely true that the Holy Spirit's power and presence is needed to do good, and lack of good is lack of Spirit, but that is not Matthew's emphasis here specifically. His emphasis here is the doing of good or lack thereof, and hence, that is what the oil represents. It represents one's works of love.
Again, if one thinks Matthew is talking about justification, then that will be an alarming statement; but that is not his purpose. It is his purpose to say that those who have placed themselves under Christ's Lordship in faith do the things that evidence His Lordship (that should seem obvious, should it not?).
Hence, immediately after the Parable of the Virgins in the Parable of the Talents, which also represent the same thing (this is not a passage teaching us about investing our money, duh). The talents are like the oil, we are given opportunities to do good, and those who truly seek the will of the Master attempt to use those opportunities to do good. In other words, Christ has done good to us in His offer of salvation to us, as we are given opportunity to do good to others, we can either squander the good done to us selfishly or share it with other Christians, multiply it within the community. Hence, as Christ says in John, they will know that we are His disciples by our love for one another. John tells us further in his first epistle that this love is seen in tangible terms (i.e., not just in feelings and words spoken, but also in deeds of doing good/preserving our fellow Christians' lives who are in need).
So the question becomes, Do you have sufficient oil? The parable warns us that Christ will return in one way or another and we will not have the opportunity to fill our lamps in the end. Our opportunity to do good is today, not tomorrow. Today He calls us to be faithful. Today He calls us to love. Today He calls us to the cross and to sacrifice and to the death to the self that doesn't want to give up its time and resources that it has preserved for itself.
The answer to an unfilled lamp is not going out and doing good in one's own power. We know from the rest of Scripture that provides context to the Gospel of Matthew itself that apart from Christ we can do nothing. It is abiding in Christ, who is the vine of true life, that gives us the love and power to overcome the fear of losing the self and gives us a love and fear to do what is good to others who represent Christ to us (as the phrase "the least of these" refers to the most insignificant of Christians--thus showing that any and every Christian represents Christ Himself). What we do to other Christians, we are doing to Christ (cf. Christ's statement to Paul, "Why are you persecuting Me," when Paul had never come into contact with Christ before personally). Would not the man who loved Christ give up all that he had if he saw Christ in need? But the man who does not figures someone else will do it, and makes excuses as to why he's justified in keeping these resources for himself.
Don't be caught with low oil in your lamp. Don't be caught without true faith. Ask yourself this question, and judge thyself, not thy neighbor, "If I were to die and come before Christ this very moment, would my life evidence true faith or false faith? Would it show my works to be an abstention from evil and the doing of good, or would it evidence nothing but sins of commission and omission?" Then please do what Christ commands in His very first opening statement of His preaching ministry within the Gospel of Matthew, "Repent, for the kingdom of God is at hand" (4:17).