Q. Can a mantra really neutralize the sins already done or does it only give the courage to bear the pain (result)?
A. The Holy Name of God can actually eradicate the reactions for all the sins already committed.
Not only that, scriptures state that when properly chanted, hari-nama can destroy more sins than one can possibly ever commit.
In order to derive the full benefit of chanting — it can even burn up the residual tendency in the heart to commit further sins! — chanting must be taken up properly. One should carefully avoid the ten kinds of offenses in chanting, especially the mentality that one can continue sinning and that it will be adjusted on the strength of chanting. Conversely, an attitude of surrender, sincere repentance and adherence to a theistic way of life are favorable and quickly help to perfect one’s chanting.
Chanting gives immediate relief from the blazing reactions of past sins, and quickly ushers in auspiciousness – this can be tangibly experienced in our lives. Yet, as long as we are in this material ocean, its waves and whirlpools will not cease; that is to say that by taking up chanting it is not to be expected that there will be never be another difficulty/pain/reversal in one’s life. But these do not have the same disturbing effect upon those who take firm shelter in the Holy Name. In that sense, one could say that chanting gives the spiritual strength to tolerate material disturbances, but this is not the same as suffering that comes from past sins. And the Holy Name ultimately delivers one from this treacherous material existence altogether