Hadhrat Abu Hurayrah رضى الله تعالى عنه narrates that the Messenger of Allah said: One who stands up for worship, in Ramadhan, with faith, and in expectation of reward, will be forgiven for his past sins. (Bukhari/Muslim)
The phrase, ‘with faith, and in expectation of reward’, means that a person should acknowledge the reward for fulfilling this command of ALLAH TA’ALA, and he should perform it with sincerity and in anticipation of reward.
Forgiveness of sins, refer to minor sins.
As for major sins, Tau-bah and Is-tigh-faar, (Repentance and seeking forgiveness) is required. If a person is fortunate, to have no minor or major sins, then it will be a means of elevatiing his rank in Paradise.