Read the series so far.
This seems to be a simple question, with a simple explanation. I’m willing to bet when you read this question, you immediately started thinking of sin as an action in contradiction to an established norm. You aren’t alone – I did the same thing. We instinctively answer this question as if sin is an act. Even the Merriam-Webster dictionary, the closest thing we have in America to a recognized lexical standard, defines sin as, “an offense against religious or moral law.”
It is true that a sin is a transgression against a moral law. From the Christian worldview, the very idea of objective morality, and the universal human ability to differentiate between the concepts of “right” and “wrong” are proof that:
- there is a Creator,
- we are His creatures – created in His image,
- He defines morality and the concepts of “good” and “evil,” and
- all human beings are subject to His rule and, therefore, His law.
But, that’s not the whole story. It isn’t enough to craft a definition based on external actions and call it a day. Is sin just about externalism? Is it possible to think about something, and commit a sin? Is temptation still a sin, because it’s purely an internal lust? To get down to brass tacks, consider this:
- Can you lust after a co-worker, as long as you don’t act on the thought?
- Can you plan to murder the nosy neighbor next door, even if you don’t ever carry out this dastardly deed?
- Can you pretend to be nice to a Christian brother, while inwardly you hate him?
If sin is simply an outward action, the answer to each of these is, “Yes!” Unfortunately, some popular theology texts do define sin as externalism. Consider these examples:
- Charles Ryrie: “[S]in is missing the mark, badness, rebellion, iniquity, going astray, wickedness, wandering, ungodliness, crime, lawlessness, transgression, and a falling away.” This is not really a definition at all; it’s a list! But, do you notice how these descriptions are more about external action than anything else?
- Emery Bancroft: He defines sin as (1) missing the mark of the divine standard, (2) a lapse from God’s requirement, (3) a perversion of what is right, (4) a passing over of the boundaries of God’s law, (5) an affront to God, (6) unfaithfulness, (7) an offense, (7) a failure in duty and (8) disobedience. Again, this isn’t really a systematic definition at all – it’s a redundant list.
Back to externals – is sin more than an act? In the Sermon on the Mount, Jesus proclaimed the Old Covenant law as it was meant to be understood. It was not meant to be a checklist; it was a Covenant to be obeyed from the heart. This is why Jesus said:
You have heard that it was said to the men of old, ‘You shall not kill; and whoever kills shall be liable to judgment.’ But I say to you that everyone who is angry with his brother shall be liable to judgment; whoever insults his brother shall be liable to the council, and whoever says, ‘You fool!’ shall be liable to the hell of fire. (Matt 5:21-22).
You might not really shoot your nosy neighbor (or his annoying dog) twice in the chest with your trusty 9mm, but if you thought about it, you’re just as guilty. I’m being slightly silly, but you get the point. Here is a more pedestrian example:
You have heard that it was said, ‘You shall not commit adultery.’ But I say to you that everyone who looks at a woman lustfully has already committed adultery with her in his heart. (Matt 5:27-28).
Yes, it is true you didn’t sleep with your co-worker. But, you thought about it. A lot. You are just as guilty.
It seems as if It seems sin is much more than mere action. Behold this good definition of sin from a conservative Baptist theologian:
Sin is any lack of conformity, active or passive, to the moral law of God. This may be a matter of act, of thought, or of inner disposition or state. 
There is a lot which could be written from this, but I’ll focus on a few components:
- Sin is an action
- It is also a thought
- It is also a matter of status (i.e. disposition or state)
The last bit is particularly important. You can commit a sinful action. You can think a sinful thought. But, sin is also described in Scripture as a state of being. “Acts of sin spring from a principle or nature that is sin.” We are born by nature as children of wrath, which means we’re born as sinful people, in active rebellion against our Creator. As the 1833 New Hampshire Confession of Faith put it:
We believe that man was created in a state of holiness, under the law of His Maker; but by voluntary transgression fell from the happy and holy state; in consequence of which all mankind are now sinners, not by constraint but choice, being by nature utterly void of that holiness required by the law of God, wholly given over to the gratification of the world, of Satan, and of their own sinful passions, therefore under just condemnation to eternal ruin, without defense or excuse.
Consider also the Apostle Paul’s words:
What then? Are we to sin because we are not under law but under grace? By no means! Do you not know that if you yield yourselves to any one as obedient slaves, you are slaves of the one whom you obey, either of sin, which leads to death, or of obedience, which leads to righteousness? But thanks be to God, that you who were once slaves of sin have become obedient from the heart to the standard of teaching to which you were committed, and, having been set free from sin, have become slaves of righteousness. (Rom 6:15-18).
Sin here is not an abstract action. It is a state of being. In this passage, it is a taskmaster people are naturally enslaved to – a master who only brings death. In contrast, God is the good master who distributes righteousness to His slaves.
So, when you think about sin, remember it is much more than an action. It is also a thought in your mind and heart. It is also a status which brings eternal damnation and everlasting condemnation, unending hostility and anger from the Holy God who made you, fashioned you, sustains you and calls you even now to repentance and faith in His one and only Son, Jesus Christ.
But, is there something even more fundamental, more basic, to the idea of “sin?” There is. For, now, however . . . ciao.
 Merriam-Webster Collegiate Dictionary, 11th ed. (Springfield, MA: Merriam-Webster, 2003), s.v. “sin,” 1a.
 Charles Ryrie, Basic Theology: A Popular Systematic Guide to Understanding Biblical Truth (Chicago, IL: Moody, 1999), 243-244.
 Emery Bancroft, Christian Theology, second revised ed. (Grand Rapids, MI: Zondervan, 1976), 218-226.
 See Leon Morris, The Gospel of Matthew, in PNTC (Grand Rapids, MI: Eerdmans, 1992), 112-113. D.A. Carson quibbled a bit, and wrote, “The contrast between what the people had heard and what Jesus taught is not based on distinctions like casuistry versus love, outer legalism versus inward commitment, or even false interpretation versus true interpretation, though all of them impinge collaterally on the text. Rather, in every case Jesus contrasts the people’s misunderstanding of the law with the true direction in which the law points, according to His own authority as the law’s ‘fulfiller’ . . . (Matthew, in EBC, vol. 8 [Grand Rapids, MI: Zondervan, 1984], 148).
 Millard Erickson, Christian Theology, 2nd ed. (Grand Rapids, MI: Baker, 1998), 596.
 Henry C. Theissen, Lectures in Systematic Theology (Grand Rapids, MI: Eerdmans, 1949), 244.
 1833 NHCF, Article 3, quoted in William L. Lumpkin, Baptist Confessions of Faith, revised ed. (Valley Forge, PA: Judson, 1969), 362.