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Well, there is a strong tendency in Catholics to trust authority. I don't recall which Commandment can be interpreted to mean "Thou Shalt Trust The Authorities". Another question I'll have to ask my priest.
At my first Confession, when I was through with the actual confession part, I asked the priest if speeding is a sin, as many believe it is -- as in disobeying civil authorities -- and that priest rolled his eyes, which confirmed my belief that speeding, as long as you are not endangering another, is not to be considered a sin.
But no, the priest is not likely to withhold absolution in the case of the penitent who has not turned himself in to the authorities. What might be more likely is that the priest could terminate the confession and boot the person out of the confessional before the penitent has the chance to perform the Act of Contrition -- a sincere Act of Contrition is necessary in order to merit absolution. The penitent must tell God how sorry he is for committing the sin and promise to make every effort not to sin again.
Also, it is up to the priest's discretion to withhold the Sacrament of the Eucharist -- Holy Communion -- from that penitent. If someone has every intention of going on committing a mortal sin, they are not in a fit state to receive the Sacrament.
PS -- I'm still a fairly new Catholic, and my opinion is certainly not to be taken as Catholic law.
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