What is the opposite of sin?

Under the new covenant. I would think that "Not accepting Jesus" as sin.

It is blasphemous to say that He cannot forgive me of such and such act. Nothing is impossible for Him to forgive, except we accept Him and His sacrifice.

or I think I understood your question wrong.

If certain deeds are sins because they are done in disobedience of God's law, then one plausible suggestion is that Faith is the opposite of Sin:

Faith is the root of obedience; the lack of faith is the root of disobedience. If we totally believed that obedience always worked to our blessedness, we would not disobey. Only because we must pray “Lord, I believe, but help my unbelief” do we sin.

Saint Paul contrasts faith with sin when he says, “Whatever is not of faith is sin." We usually think of sin as the opposite of virtue, and faith as the opposite of doubt. But virtue is a moral term, and doubt is an intellectual term. The opposite of moral virtue is moral vice, and the opposite of intellectual doubt is intellectual belief. Faith is deeper than either moral virtue or intellectual belief. Sin is deeper than either moral vice or intellectual doubt. Faith is a fundamental Yes to God with the center of our being, and sin — the state of sin as distinct from particular acts of sin — is the fundamental No to God with the center of our being. Faith is the opposite of sin. Faith is to sin what light is to darkness.

This is talking about sin and faith as "states", distinct from particular acts. By analogy with "acts of sin", we have "acts of faith". Particular sins result from sin in general and original sin (the disobedience of Adam and Eve). Likewise, particular "acts of faith" result from faith. One passage in the Bible that discusses these acts is James 2:14-26, and it refers to these as "works":

26 For as the body without the spirit is dead, so faith without works is dead also.

• Life -> breath, pulse, etc.
• Sin -> individual sins
• Faith -> acts of faith

Each sin comes from Original Sin, and each act of faith or good work comes from Faith.

For the verb which opposes to sin, the Catholic answer is to obey. More specifically, to sin in the general religious sense opposes to obey God the Father.

Sin is an offense against God: “Against you, you alone, have I sinned, and done that which is evil in your sight.” Sin sets itself against God’s love for us and turns our hearts away from it. Like the first sin, it is disobedience, a revolt against God through the will to become “like gods,” knowing and determining good and evil. Sin is thus “love of oneself even to contempt of God.” In this proud self-exaltation, sin is diametrically opposed to the obedience of Jesus, which achieves our salvation. (Catechism of the Catholic Church, 1850)

This form of to obey is equivalent to to love, which in Catholicism is understood as to seek the good of the other as other.

The corresponding noun then is obedience [to God] or agape/love.

For your second set of fill-in-the blanks ...

To obey [God] is less ambiguous, but also less "full" I think:

• What Cain did was a sin, but what Joseph did was obeying [God] / an act of obedient [to God].
• Offering up a sacrifice, visiting someone in prison, and praying are all examples of obeying [God].
• Obedience [to God] is an utterance, a deed, or a desire in accord with the eternal law.
• Obedience [to God] is the keeping of God's law.
• Obedience [to God] is nothing more than a morally good act, an act in accord with reason informed by the Divine law.

To love is fuller in meaning when properly understood, I think.

• What Cain did was a sin, but what Joseph did was to love / an act of love.
• Offering up a sacrifice, visiting someone in prison, and praying are all examples of love.
• Love is an utterance, a deed, or a desire in accord with the eternal law.
• Love is the keeping of God's law.
• Love is nothing more than a morally good act, an act in accord with reason informed by the Divine law.

In order to know what the opposite of sin is, we must first understand what sin itself is. Sin is defined as a violation of God's law or rebellion against God or His moral purity.

1 John 3:4 (KJV)

Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.

The opposite of sin is, then, the state of obedience to God's law, acknowledgement of His rule, and conformity to His moral purity.

So, basically, we choose either to sin or not to sin, to violate God's law or be obedient to God's law, to rebel against God or recognize Him as God, to act in impurity or in purity or in righteousness.

Sin comes from the Greek Hamartia, which carries with it the idea of an archer missing his mark. The opposite of that would be to hit the mark- to succeed.

I would argue thus that obedience would be the opposite of sin, as it "hits the mark" to which we aim.

Alternatively, telos - the purpose or aim for which we are destined would be pretty good too.

Augustine in particular was well known for describing evil as not a thing in and of itself, but rather as a lack of good. It is the lack of substance to the thing that is its nature, not an essence of its own. The same could be said of sin - that it is a lack of the nature of God's desire for us that characterisizes Sin. As such, the opposite of Sin would be the nature of God evident in the life of the person in question.

And, btw, it totally is fair to say that the opposite of Sin is love- for as Jesus himself said, "if you love me, keep my commandments."