... With faith and confidence, your heart being firm and certain, without any doubt whatsoever, that Hashem delights in kindness, is gracious and merciful and abundantly forgiving, that the instant somebody asks Hashem for forgiveness, Hashem forgives him.
... In every Shmoneh Esrei, we ask Hashem slach lanu... and we conclude “Baruch Atah Hashem, Chanun Hamarbeh Lisloach – Gracious One, Who pardons abundantly.” Now, we do not say Hashem’s Name if the obligation is doubtful, lest we are saying a brocha levatalah – saying a brocha in vain. We can thus deduce that there is no doubt whatsoever that Hashem forgave us, since we ask “slach lanu; mechal lanu - pardon us, forgive us.”
If we do not repeat and transgress again, we will be redeemed immediately, as we bless in the very next blessing, “Baruch Atah Hashem, Goel Yisroel – the Redeemer of Israel.”
==== Tanya, Igeres HaTeshuvah Perek 11
