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 ||