Some things can be considered sins at certain points in history and/or for certain people and not others, and still others are always and everywhere sins (assuming, of course, the requirements for sin are met).
I don’t know the proper terminology for this, so I will be descriptive rather than prescriptive:
Certain things can be sins for certain people. One example is the Pontifical Secret. A common use of this is when priests are notified that they will become bishops; when this happen, the Nuncio of the country tells the man, often via phone, and he is held under the strictest confidentiality not to tell anyone under the direct authority of the Pope. This only binds to the people who know, and the people who know are the ones who have to know. There are other examples of when the Pontifical Secret is used. This is an exercise of legitimate Papal authority even though “Pontifical Secret” is not explicitly contained within the Bible or Tradition as such.
Certain things are sinful in some periods and not others. For example, the Communion fast until recently in Church history used to be significantly longer than it is now, such that it actually made a difference in communicants’ conscious lives in that they had to actively make decisions about when to stop eating; now, it is very easy for many people to eat breakfast soon before going to Mass and the fast will not be broken. No longer is it a sin to eat after midnight, or more recently three hours before Mass begins, and receive Communion.
Certain things are commanded by God in some unspecific practical form which the Church nevertheless applies certain practical forms to satisfy God’s commands. The obvious example is fasting, to satisfy God’s command to do penance: the Church commands that we fast on Fridays during Lent, and all other Fridays in the year or, instead of in the latter case, to substitute something in its place. Even though God didn’t tell us, “Fast on Fridays,” He did tell us, “Do penance,” and the Church has grown a tradition, and now a law, of fasting on Fridays to meet the bare minimum of this divine command.
Still other things are always sinful, as long as the requirements for sin are met: essentially anything that violates a Commandment.
