Q - Why do you have to be Catholic to be absolved from sin in confession?
A - Thanks for the question. There are only two Sacraments of the Catholic Church that, under normal circumstances, are validly received by non-Catholics.
The second is Matrimony - two baptized Christians confer the Sacrament upon one another. If one or both of the spouses is not baptized, then the marriage is still valid, but not a Sacrament.
Now, you have to be a baptized Catholic (with a few rare exceptions I talk about below) to be able to receive absolution in Confession. This is because Confession isn't just about an individual and God. It is also about our relationship with the rest of the Body of Christ - the Church. Thus, when we sin, we wound others as well. Confession (also called Reconciliation and Penance) is the ordinary way in which God forgives mortal sins AND heals the wounds caused to the Body of Christ - the Church. Thus, when we go to Reconciliation, we are healing our relationship with every other Christian, living and dead, as well as with God!
Furthermore, one must be able to say that they believe that a Catholic priest has the power to forgive sins.
If they do, then they must believe the priest was validly ordained.
If they do, then they must believe in the apostolic nature of the Bishops and in apostolic succession.
If they do, then they must believe in the power of the Sacraments, esp. the Eucharist.
If they do, then they need to be Catholic.
Therefore, only Catholics can go to Confession. But, it is for everyone, because everyone is called to join the Church.
This is why Confession is reserved for Catholics alone, except in rare circumstances, including the following outlined in the Catechism:
“When, in the Ordinary's judgment, a grave necessity arises, Catholic ministers may give the sacraments of Eucharist, Penance, and Anointing of the Sick to other Christians not in full communion with the Catholic Church, who ask for them of their own will, provided they give evidence of holding the Catholic faith regarding these sacraments and possess the required dispositions.” -CCC 1401Then there is also an exception given to baptized non-Catholics who are in RCIA and about to enter the Church. This is allowed in order to prepare the person spiritually before they receive Confirmation and the Eucharist.
I hope this helps.
**Is Confession To a Catholic Priest Necessary or Can You Go Straight To God?
**What Does Being Contrite Mean?
**How Often Should I Go To Confession + Other Questions.
**A Guide To Confession.