If you mean the opposite of "I committed a sin" in the sense of "I did not commit a sin", you can just say that. (Or something close like "I am blameless", "I am free from guilt/sin", etc.)

But if you mean an act that's the opposite of sin, then, to the best of my knowledge, there is no word in English for such an act. (I know a certain Sanskrit teacher who has often observed in speeches and the like that while English readily has a word "sin" for Skt pāpa, an act against dharma or which brings bad karma, it doesn't have a word for the opposite puṇya — and if someone had pointed out a word to him he'd probably have stopped saying it. :p) I tried a few dictionaries and they give approximations like "the good or right, virtue, purity, good work, meritorious act, moral or religious merit" or adjectives like "good, meritorious, virtuous, righteous, just". The fact is that different cultures have different sets of concepts that are reflected in their languages.

So your best bet is probably to use approximate adjectives, like "I did something good" or "I performed a meritorious/virtuous act". [Or, of course, since English does not hesitate to use words from other languages, you could use words like the suggested Hebrew mitzva or Sanskrit punya, depending on your audience.]

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).