I’m sorry, but they are not synonymous terms in English, and until you prove otherwise, they aren’t synonymous in Church teaching, either.
Church uses the terms as synonyms.
And what does one HAVE to confess? Grave sins (what is also called mortal sins).
I understand very much the confusion here --it pops up here from time time
One does not commit a grave sin --even if it is grave in matter (objectively grave) --if one lacks the needed knowledge or deliberate consent.
But as to theological terminology it is important to note that the three terms are used to mean the same sin.
A Few Examples of the Church interchanging the terms: They are
synonyms.
CCC1385: “Anyone conscious of a **grave sin **must receive the sacrament of Reconciliation before coming to communion.”
From: The Compendium of the Catechism issued by Pope Benedict XVI
"304. Which sins must be confessed?
1456
All
grave sins not yet confessed, which a careful examination of conscience brings to mind, must be brought to the sacrament of Penance. The confession of** serious sins **is the only ordinary way to obtain forgiveness." (see also Canon Law …the term is used there too).
- What is required to receive Holy Communion?
1385-1389
1415
To receive Holy Communion one must be fully incorporated into the Catholic Church and be in the state of grace, that is, not conscious of being in
mortal sin. Anyone who is conscious of having committed a** grave sin** must first receive the sacrament of Reconciliation before going to Communion.
(note that one used BOTH to mean the same thing in the same paragraph… for
variety in use of language in writing.)
“However, it must be remembered that the church, guided by faith in this great sacrament, teaches that no Christian who is conscious of **grave sin **can receive the eucharist before having obtained God’s forgiveness.”
Pope John Paul II Reconciliatio et Paenitentia 17
And in a speech from Pope John Paul II
“The sacrament of Penance is meant to take away personal sins committed after Baptism: first of all
mortal sins, then venial. If the penitent has committed more than one mortal sin, they can only be remitted all at once. In fact, the remission of **serious sin **consists in the infusion of the sanctifying grace which has been lost, and grace is incompatible with any and every serious sin. Venial sins are to be regarded differently…”
and in ECCLESIA DE EUCHARISTIA
“If a Christian’s conscience is burdened by
serious sin, then the path of penance through the sacrament of Reconciliation becomes necessary for full participation in the Eucharistic Sacrifice.”
And Pope Benedict XVI in his Catecheses with Children said:
"Only in that case, when you are in a state of
“mortal” sin, in other words,
grave (sin), is it necessary to go to confession before Communion. This is my first point. "
vatican.va/holy_father/be…ildren_en.html
grave sin = mortal sin = serious sin