The chanting itself purifies us and as the heart becomes purified, the Holy name is revealed to us as being non-different than Lord Krishna.
Our spiritual progress depends on service attitude and our sincere chanting.
The holy name of Krishna is so attractive that anyone who chants it — including all living entities, moving and nonmoving, and even Lord Krishna Himself — becomes imbued with love of Krishna. This is the effect of chanting the Hare Krishna maha-mantra.
Chaitanya Caritamrta, Antya Lila 3.268,
This process of chanting is the best process of self-realization in this age; therefore Lord Caitanya preached it so nicely for the benefit of all humanity.
Srimad Bhagavatam 3.24.35,
One who chants the holy name of the Lord is immediately freed from the reactions of unlimited sins, even if he chants indirectly [to indicate something else], jokingly, for musical entertainment, or even neglectfully. This is accepted by all the learned scholars of the scriptures.
Srimad Bhagavatam 6.2.14,