In the religion of Islam nobody has the permission to disclose their sins (small or big) and unveil their secrets to anybody. The reason why is, to begin with, man’s honor and respect is so valued and significant he should only confess to his lord as a gesture of repentance and asking for forgiveness, for one will only obtain more respect and honor when he confesses to Allah. This is something solely confined to God; nobody else can compensate the respect that has been lost (by confessing to one’s sins) let alone strengthening it. Second of all, from the Islamic perspective and based on the tawhidi (monotheistic) standpoint the sole cause and reason behind everything in this world is Allah; without his consent and approval nobody can make the slightest change in anything. In addition to that, nowhere in Islam has it been stated that a specific person or group of individuals posses the authority to grant forgiveness.
On the other hand what is referred to in Islam as shafa’ah (intercession) of the Owliya’ullah (friends of God) does not suggest that they have the ability to directly intervene in the act of forgiving, but rather act as intermediaries between the sinner and Allah. Moreover, the shafi’ essentially intercedes on behalf of those who have been able to acquire the necessary preparation and groundwork for being pardoned. Therefore shafa’ah is in no way a means of making money and reaching personal interests.
There are many ways to receive forgiveness and pardon. We will mention a few of them here:
1. Repentance and rededicating oneself to the path of God (contingent upon its conditions).
2. Doing outstanding good deeds which cause forgiveness of sin.
3. Abstaining from the cardinal sins; which in turn results in the forgiveness of the smaller sins.
4. Tolerating the tribulations and misfortunes of this world, difficulty of the inter-world (Barzakh), and the initial trials of the Day of Judgment will help to eradicate his sins.
5. Intercession. Of course it must be taken into consideration that intercession is contingent upon the change and transformation of the sinner from his previous state and him having the necessary capacity for forgiveness and remission; in order for him to become cleansed from sin.
6. Divine forgiveness for individuals who are worthy and have met the necessary conditions; meaning for the “believers” (Mu’mineen) who have had some shortcomings or incompetence in their actions.
In Islam one cannot disclose his sins (small or big) to anyone; he must keep his secrets (regarding his sins) to himself. From Islam's perspective the self-respect a person has for himself is so significant that he can only confess his sins before God in the sense of asking for forgiveness. Not only will he not lose his respect but by confessing before Allah he will gain more. This is something solely restricted to God, for no other person even if he were to be a trustworthy person, is capable of returning one’s respect (that has been lost when he confesses to that person), let alone increasing it. On top of that, based on the tawhidi (monotheistic) viewpoint in Islam the sole cause for everything in this world is God  and no one can do anything without his permission especially granting forgiveness and pardon which has not been assigned to any individual or character in Islam other than Allah (swt) himself.
As for what Islam introduces as shafa’ah  (intercession) of the Owliya’ullah (friends of God), first of all, it is their mediating for the sinner’s forgiveness, not that they directly grant forgiveness themselves. In his book, Shahid Motahhari says: "Shafa’ah is the same thing as divine forgiveness; when it is attributed to Allah (swt), it is forgiveness, and when attributed to the intercessors and mediators, it takes on the name of shafa’ah." 
Second of all: Shafa’ah isn't given to just anyone, it is confined to Allah and is by his will bestowed upon those who by obedience and righteousness have manifested godlike values in their lives such as the Prophets, their successors, the Imams (A.S.), religious scholars, martyrs and true believers.
Third of all: The shufa’aa’ (interceders) can only do shafa’ah for those that are qualified to be granted it; it isn't to be used as a means of reaching material objectives and personal desires.
The shufa’aa’ (interceders) do shafa’ah (intercession) for those whom God has given permission; "They cannot intercede except for those whom He pleases."  Thus not everybody is able to be interceded for.
In reality, the concept of shafa’ah revolves around the principle of change, transformation and amendment of the “interceded individual” from his previous state which means that the person who is being interceded sets the groundwork and preparation so that he may prevent an undesirable chastisement. 
Therefore in Islam, both the interceder and the interceded must acquire certain qualities and be worthy of their granted positions and shafa’ah is only done by Allah's will and desire and through its specific set of mediators.
However in Catholicism, virtually every one of these conditions is being neglected. Not to mention that the past history of the Catholic Church in the mediaeval ages clearly portrays the extreme deviation that took place after getting hold of such ranks and authorities and misusing them as a means of reaching personal goals and obtaining other material interests.
Historians along with knowledgeable and enlightened Christians have endlessly condemned the behavior that took place at that particular period. The Church and the fathers of the church were particularly misled from their true religious duty, some of which can be unfortunately seen in the present day. A few that can be pointed out are: financial corruption  , moral corruption  , interrogating people’s beliefs, torturing and punishing the opposition, selling and dealing forgiveness.
Selling forgiveness by the priest first started when the church passed a rule that as atonement of their sins the penitents were to pay a certain amount of money, but along with that the church would also collect funds from the same person which gradually turned into an essential condition for anyone who wanted to confess and be pardoned from sin. This practice continued to the extent where confession became a mere formality and simply a custom lacking any true substance; its main objective being to make money. 
From numerous verses of the Holy Quran we can gather that there are various ways of attaining forgiveness and pardon of sin; of which here we will briefly mention some of them:
1. Repentance and rededicating oneself to the way of Allah, with sincere regret from past sins with a conscience decision to refrain from future sin and practical compensation of bad deeds by doing good deeds. 
2. Doing extraordinary good deeds which cause forgiveness of bad deeds, just as it states in the Quran: “Indeed good deeds efface misdeeds.” 
3. Abstaining from cardinal sins, which in turn cause the lesser sins to be forgiven. “If you avoid the major sins that you are forbidden, We will absolve you of your misdeeds, and admit you to a noble abode.”  , 
4. Tolerating the tribulations and misfortunes of this world will lighten the burden of sins of the believers; just as the affliction in the inter-world (Barzakh), and the initial trials of the Day of Judgment will help to eradicate his misdeeds. 
5. Divine forgiveness
is inclusive of worthy individuals; meaning the believers who, in their actions have had some shortcomings or have been contaminated with misdeeds. If these individuals are embraced by divine pardon then they will join those going to heaven, and if not then they will enter into the hellfire; however that will not remain their [permanent] place and station and they shall not be abandoned there forever.
 لا مؤثر فی الوجود الا الله
 Shafa’ah means for someone to mediate between God and another person, in order to make some good reach him or to prevent something bad from happening to him, in this life or the next.
 Mutahhari, Morteza, Majmu’eh Asar, vol. 1, pg. 259.
 For further information, see: Question 5964 (The concept of intercession in Islam، website:6175) and Question 124 (Intercession and divine consent).
 The church gained much wealth via different means during this period, not to mention through illegitimate ways, such as accepting bribes, forging documents, etc.
 They had forgotten their mission to serve the people and were busy with debauchery and drinking and feasting, distancing from moral virtues. Erasmus’ saying in this regard is very interesting.
 Adopted from Question 299 (Proof for the distortion of the Bible), you can refer to this question for further information.
 “ هُوَ الَّذِی یَقْبَلُ التَّوْبَةَ عَنْ عِبادِهِ وَ یَعْفُوا عَنِ السَّیِّئاتِ ” Shura:25.
 “ إِنَّ الْحَسَناتِ یُذْهِبْنَ السَّیِّئاتِ ” Hud;114.
 “ إِنْ تَجْتَنِبُوا کَبائِرَ ما تُنْهَوْنَ عَنْهُ نُکَفِّرْ عَنْکُمْ سَیِّئاتِکُمْ وَ نُدْخِلْکُمْ مُدْخَلاً کَریماً ” Nisa:31.
 Amuzesh Aqa’ed, Misbah Yazdi, pg. 477.
 Amuzesh Aqa’ed, Misbah Yazdi, pg. 481.
 Tafsir Nemouneh, vol. 26, pg. 111.