"Turn to me and be saved,
all you ends of the Earth;
for I am God, and there is no other.
By myself I have sworn,
my mouth has uttered in all integrity
a word that will not be revoked:
Before me every knee will bow;
by me every tongue will swear"
God is self existent and self sufficient. God has life in and of himself. Everything God does is an act of grace because God does not have to do anything. He is perfect and content as he is and everything he does is thus "for his good pleasure". (Eph 1) God can never fail, and thus his grace never fails.
At any given moment, we are always already immersed in God's grace, our own existence a manifestation of it. However we do not realize it just as a fish does not understand that it is wet. There is only One God by which we can be saved for only one exists and as surely as the LORD lives, he is God over all people. It is only when we recognize and confess grace through the revelation of God that we are brought to faith by God and thus salvation.
He swears by himself because there is nothing greater to swear by. He is who He is and the only prerequisite for his being is his being the great unchanging "I AM" God's existence is an essential characteristic. He cannot "not be" for he is existence in and of itself.