Bahá'í views on sin
Bahá'ís consider humans to be naturally good, fundamentally spiritual beings. Human beings were created because of God's immeasurable love for us. However, the Bahá'í teachings compare the human heart to a mirror, which, if turned away from the light of the sun (i.e. God), is incapable of receiving God's love. It is only by turning unto God that the spiritual advancement can be made. In this sense, "sinning" is to follow the inclinations of one's own lower nature, to turn the mirror of one's heart away from God.
One of the main hindrances to spiritual development is the Bahá'í concept of the "insistent self" which is a self-serving inclination within all people. Bahá'ís interpret this to be the true meaning of Satan, often referred to in the Bahá'í Writings as "the Evil One".
Watch over yourselves, for the Evil One is lying in wait, ready to entrap you. Gird yourselves against his wicked devices, and, led by the light of the name of the All-Seeing God, make your escape from the darkness that surroundeth you. — Bahá'u'lláh 
The Bahá'í concept of God is both just and merciful. God is seen as being "He Who forgiveth even the most grievous of sins". Bahá'ís are meant to refrain from focussing on the sins of others, and are meant to have a "sin-covering eye". Bahá'ís are also forbidden to confess their sins to others in order to have their sins removed. Forgiveness is between a person and God alone, and is thus a very personal affair.
Should anyone be afflicted by a sin, it behoveth him to repent thereof and return unto his Lord. He, verily, granteth forgiveness unto whomsoever He willeth, and none may question that which it pleaseth Him to ordain. He is, in truth, the Ever-Forgiving, the Almighty, the All-Praised. — Bahá'u'lláh 
Bahá'u'lláh taught that one should bring one's self to account each day, and be constantly concerned with self-improvement. Sin is an inevitable stumbling block, but it should not be allowing to halt one's spiritual progress. One should ask for forgiveness from God alone and then try to develop oneself through acquisition of virtues and communion with God (through prayer, fasting, meditation and other spiritual practices). There are many Bahá'í prayers for forgiveness of oneself, one's parents, and even the deceased. The Bahá'í Faith teaches that pardon can be obtained even in the afterlife and that deeds done in the name of the departed or wealth left by the departed for charity can benefit and advance their souls in the afterlife.
The Bahá'í Faith accepts the Biblical teaching that the sin against the Holy Spirit cannot be forgiven, in this world or the world to come.
The Prophets of God are manifestations for the lordly perfections - that is, the Holy Spirit is apparent in Them. If a soul remains far from the manifestation, he may yet be awakened; for he did not recognize the manifestation of the divine perfections. But if he loathe the divine perfections themselves - in other words, the Holy Spirit - it is evident that he is like a bat which hates the light. This detestation of the light has no remedy and cannot be forgiven - that is to say, it is impossible for him to come near unto God. This lamp is a lamp because of its light; without the light it would not be a lamp. Now if a soul has an aversion for the light of the lamp, he is, as it were, blind, and cannot comprehend the light; and blindness is the cause of everlasting banishment from God. — `Abdu'l-Bahá
In the end, only God can decide who is forgiven and who is not.