I would like to know if there is an opposite word for sin in English.
I mean, how could I say the opposite of
I committed a sin
other than using a negation?
English does not have a precise word for the opposite of sin in the sense you mean, so you'll have to be content with adjectives: the opposite of "I committed a sin" would be "I performed a good/virtuous/righteous/moral/meritorious act/deed". (Note that the noun forms of these adjectives won't work: goodness has a very wide range of meanings; virtue refers to qualities inhering in a person and carries no connotation of action unlike virtuous act which does, etc.)
As you're writing for an Indian audience, the word you have in mind that's already familiar to your audience (puṇya) is a perfectly good choice to use. The criterion should not be whether a certain word exists in an English dictionary or not, but whether your audience will understand the word or not.
Further thoughts: English doesn't have have a word for the opposite of sin, because sin is a religious concept, and mainstream Christianity doesn't have a concept that's the opposite of sin; neither have English speakers found it necessary (yet? :p) to invent a word for the concept. In a non-religious framework for ethics, of course, there is no such thing as sin either; though certain acts may still be called unethical or wrong or by other terms.
I'm no expert on Christian theology, but it seems that according to that framework, one is born in a state of some sin, and although one can commit further sins (acts against God's commandments), one cannot automatically reduce the effect of those sins simply by performing other good acts. Judaism has a concept of mitzvah, an act that carries out a commandment of God, which may be an opposite of sin in that sense. The concept you may be getting at, prominent in Indian religions (Hindu/Buddhist/Jain/Sikh) comes from a different model, in which there's something like a moral bank balance (karma) in which you can either lose credit through sin (pāpa, acts against some cosmic order of right and wrong) or gain credit/merit through good deeds (puṇya).
All that is not important, but if by the opposite of sin you're referring to something like the latter concept as informally understood by your audience—with a slight theological connotation as something that brings merit to the doer—then the term puṇya you were thinking of is precisely the right term to use. Using a generic phrase like "good deed" may not convey the intended meaning unless the context is understood (such as in translations etc., where "good deed" and "meritorious act" are indeed used).
The closest that I can think of is virtue. The Roman Catholic Church contrasts the Seven Deadly Sins with the Seven Cardinal Virtues. However, the word is not normally used by itself to describe a particular act, only a trait (or set of traits: patience, prudence, courage, etc.). It is commonly used in the adjectival form "virtuous", so you could say:
I have committed a sin.
I have performed a virtuous act.
I've seen mitzva used.
In religious contexts that I'm aware of you can't "pay forward" on sins. Therefore, any opposite for "sin" would have to be considered as a repeal or undoing of that sin.
For "sin" used as a noun, some suitable antonyms would be "atonement" or "expiation." The drawbacks of these two terms is that they are often interpreted as acts of personal suffering rather than acts of morally good deeds.
For the verb "sin", the verb forms of the aforementioned terms, "atone" and "expiate," should be equally suitable. Another verb that can be used as the undoing of sin is "redeem," as in:
Of course, all of these terms can be used outside of the context of religious sin, so how much "oppositeness" they have with "sin" would depend on the context in which they're used.
Sin is so completely tied into religious terms and meanings that finding a direct opposite begins to imply much about how you are using the word. Opposites of sin do very much exist but most of them require a specific form of the word:
This is a sinful act / This is a holy act
I am full of sin / I am full of righteousness
But the most direct opposite comes from the old term indulgences:
In Catholic theology, an indulgence is the full or partial remission of temporal punishment due for sins which have already been forgiven. (Wikipedia)
Actually using the word in a sentence is a bit of a pain and really won't satisfy a daily opposite of sin:
I committed a sin / I was granted an indulgence
Part of the problem comes from the source of the act being performed: Humans perform sins; God or the church performs indulgences.
The next best term would be repentance:
I committed a sin / I performed a repentance
Likewise with penance:
I committed a sin / I performed penance
Absolution, protection or recovery from sin is also implied in the religious terms sanctification, justification, baptism, confession, forgiveness, holiness, righteousness. These terms hold their own chapters in Christian theology and to even summarize them here would be futile.
All of these, however, come at this term from the religious angle. If you simply want an antonym for "bad deed" you will find a similar difficultly in looking for an antonym for "crime." Any number of phrases will work but a specific word for the opposite of a crime will be hard to find because the concept of a crime doesn't really have an opposite.
I committed a legal act
I committed a good act
I committed a virtuous act
The best I can think of is referring to a character trait as the action itself:
I performed a kindness
I performed an altruism
These have mixed success.
There is a bit of a theological question here. In my religious tradition, sin is something we do (or fail to do) that brings us guilt before God, but there is nothing we can do that can remove that guilt or restore that relationship. That's why Christ came and died for our sins—to remove that guilt which we could not by our own good works, and to turn away God's wrath.
Sin is primarily a religious concept, which takes various forms. As noted above, certain religious traditions have an antonymous concept (such as mitzvah in judaism), but there is no generally accepted term, because there is no generally accepted concept.
In the judaeo-christian context, perhaps the best approach is to translate mitzvah into English; I understand that the closest translation is "duty".
Essentially, this is the wrong question to ask, because English is used by many, many sub-cultures which all have very, very different ideas about this area.
If we're using the Biblical definition of sin, then sin is transgression of God's law. Many versions translate the Greek as "Lawlessness".
Therefore, the opposite would be "lawful".
Interpreted another way, sin can also be said to be disobedience to God's law, so "Obedience" is another possible antonym for sin.
Sin can be both a verb and a noun. The opposite of the verb sin is Repent or atoned.
First I sinned, then I repented so now God loves me - yea!
The opposite of the noun form can be any of the following: good deed, help, redeemed, basically anything that means you did a good thing.
I made up for that sin by doing a good deed, now God owes me one, right?
It depends what you mean by sin.
If you are using sin to mean disobedience to one of the Ten Commandments or to 'God's laws', then the opposite must be obedience to or obeying the Commandments.
If you are using sin to mean 'doing wrong' in general, then it depends how you perceive 'right and wrong'. If you perceive them as 'black and white', i.e. one or the other, then the opposite must be right.
But if you perceive 'right and wrong' as two extremes with a whole area in the middle that is neither right nor wrong, then there can be no exact opposite. For example, it might be considered wrong (sinful) to hate your neighbour, and right to love you neighbour. But you might actually be indifferent to your neighbour, neither loving nor hating him. In that case, are you sinning; are you doing right or wrong: or are you in the middle?
I feel the opposite of Sin is " Heavenly deed". Sinners go to hell and people who does good deeds and obtain " Punya" ( Indian opposite word for Sin) will go to Heaven. Good and Ban are are set by each society differently. Some societies view Homosexuality is sin and some view it opposite. How can one define, good or bad, but what ever society or parts of the Country set is decisive factor. Some tribal areas Smoking etc, is natural and legal for them, but not in certain areas. One can not set the exact meanings for Beauty, wind or smell, as how one perceives. Good virtues or deeds is how the particular place or part of the World set the rules and view. Even the above words Heaven or Hell are also if one believes
The opposite of sin, or to be free from sin, or wrong, is to be blameless, or without sin:
Impeccant a. Sinless; impeccable. --Byron. [1913 Webster]
Notably, sin is a religious and not a secular, and the concept is based on being in full communion with God. The action of doing good deeds does not make one without sin, not addresses the communion itself. Perhaps this contradiction is best understood in the example of a mobster who kills without impunity but yet is adored for their generosity within their community