This passage contains metaphorical usages of the heart, the eyes, and light and darkness, teaching us about spiritual blindness. Jesus' basic theme is our focus—the things we set our eyes or hearts on or the thoughts to which our minds continually return.
We should understand the word “mammon” broadly and not limit it just to money. Because Jesus presents only two options here, we can define mammon as “anything other than God”! Mammon can be anything “under the sun” and thus include countless things that we cannot necessarily hold in our hands yet are still of the flesh. We may feel good about ourselves for being content with a middle-class lifestyle and not pursuing wealth. However, if our hearts pursue praise, popularity, position, power, or prestige, we are still serving mammon. These are still cares of the physical life rather than the conduct of the new life.
Jesus teaches that our clarity of vision depends on our focus—on what gets our attention. Whether healthy or diseased, our spiritual eyesight is directly related to what we treasure and whom or what we serve. Having the wrong treasure or serving the wrong master equates to having a bad eye and walking in darkness. His illustrations mean that blinding ourselves can be as simple as letting God slip from our view or not retaining God in our knowledge, as Romans 1 mentions.
We stumble and sin when we lose our focus on God and what matters to Him. Then, our understanding begins to regress, if only a little. We may start down that pervasive path of sinning, further damaging our understanding, and sinning again.
Perhaps this scenario seems overblown or excessively dire. However, it seems so only because we think of it like a sped-up, time-lapse video of a seed that germinates, grows, blossoms, and fades in a matter of seconds. In real life, this process of darkening our own eyes may take substantially longer, during which time we face many opportunities to choose differently.
Jesus' brother, the apostle James, also describes this process quite simply, beginning with an enticement or desire (James 1:13-15). The temptation is the equivalent of the earthly treasure, the mammon, taking one's eyes off God. He writes, when “desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death” (verse 15).
This process sounds like it progresses rapidly, and in some relatively rare instances, like with Ananias and Sapphira, death can result in a matter of hours. In other cases, as with Judas, that wrong focus—a form of blindness—may persist for years before the internal suppression of truth breaks out in an act that God may allow to happen as a judgment. That does not mean we are lost, unlike Judas. It means we could have taken a better road and kept ourselves—and maybe others—from extra grief and regret.
