Bhagavad Gita Verse 12, Chapter 18

anishtamishtam mishram cha trividham karmanaha phalam |
bhavatyatyaaginaam pretya na tu sannyaasinaam kvachita || 12 ||
 
The three fold fruit of action, undesirable, desirable and mixed, goes after death to the atyaagees, but never to the sanyaasis.
 
anishtam : undesirable
ishtam : desirable
mishram : mixed
cha : and
trividham : three fold
karmanaha : actions
phalam : fruit
bhavati : goes
atyaaginaam : atyaagees
pretya : after death
na : not
tu : but
sannyaasinaam : to a sanyaasi
kvachita : ever
 
Shri Krishna elaborates on the theory of karma in this chapter. Every action has a reaction, and this reaction is also known as the phala, result or fruit of the action. When we cross the road to catch a bus, we could get one of three results. We could catch the bus, we could miss the bus, or we could get into a packed bus with no room for seating. The result could be desirable, undesirable or mixed. This is a law of the universe, no different than the law of gravity. The fruit always comes to the performer of the action, with the same certainty as the bill collector comes to collect a debt, per the words of Sant Jnyaneshwar.
 
Any fruit that is found in nature contains seeds. These seeds can create trees that can generate many more fruits. Similary, every fruit that results from an action has the potential to create innumerable desires that result in even more actions. This infinite chain of action, reaction, fruit, desire and action is nothing but bondage or samsaara. One who is an atyaagi, one who has not given up attachment to fruits of action, is bound in this chain. By practicing karmayoga, by giving up attachment to the fruit of action, we can break this chain.
 
Now, let us proceed to the next phase of karma yoga. As long as we hold the notion that we are performing all actions, we cannot proceed further in the journey of a seeker. Performance of action is falsely superimposed on the eternal essence which is beyond all action, beyond all change. The second chapter made it very clear that there can be no change in the aatmaa, the eternal essence. Unless we fully internalize the notion that our "I", our self does not perform action, we will remain entrenched in samsaara. Shri Krishna says that only a sanyaasi, only one who sees that Prakriti really performs actions, can destroy current, past and future fruits of action. This topic is elaborated in the following shlokas.

Bhagavad Gita Verse 28, Chapter 17

ashraddhaya hutam dattam tapastaptam kritam cha yat |
asaditityuchyate paartha na cha tatpretya no iha || 28 ||
 
Whatever is sacrificed, donated or done, and whatever penance is performed, without faith, it is called asat, O Paartha. It is neither here nor after death.
 
ashraddhaya : without faith
hutam : sacrificed
dattam : donated
tapaha : penance
taptam : performed
kritam : done
cha : and
yat : whatever
asat : asat
iti : in this manner
uchyate : is called
paartha : O Paartha
na : neither
cha : and
tat : that
pretya : after death
no : nor
iha : here
 
In the previous shloka, Shri Krishna asserted that any action performed with the steadfastness in Ishvara, with constant memory and faith in Ishvara, automatically becomes a saattvic action. Furthermore, with the application of the purifier Om Tat Sat, it becomes a means towards liberation. Here, such kind of action is compared with an action that is performed without any faith or steadfastness whatsoever. Action performed without any faith is called as asat, which literally means non reality or devoid of reality.
 
In our daily life, we can immediately tell the difference between one who puts their heart and soul into their actions, and one who is just going through the motions. We ourselves have instances where we love an action so much that we put everything in it, we get lost in it, and some other actions where we are acting like mechanical machines, like robots. Shri Krishna says that any action, any sacrifice, penance or charity performed without faith, without our soul in it, becomes a worthless action. Forget liberation, it will not even yield a result here, on this earth.
 
With this shloka, Shri Krishna concludes the seventeenth chapter on the three types of faith. He says that there are three types of devotees based on the texture of their faith, and are categorized as saattvic, raajasic and taamasic. In order to make ourselves fit for liberation, we should cultivate saatvic faith and eliminate the other two types of faith. This will happen only by consuming saattvic food and performing saattvic sacrifices, charity and penance. To ensure that our saatvic actions are free of errors and defects, we should use the purifier Om Tat Sat while performing the actions.
 
om tatsatiti shreematbhagavatgitasupanishadsu brahmavidyaayaam yogashaastre shreekrishnaarjunsamvade shraddhatrayovibhaagayogo naama saptadashodhyaayaha || 15 ||