John 1:16 says: From his fullness we have all received, grace upon grace.
This is a point I have made before in commenting upon various verses from John the Theologian's biblical works. His theology is very much a kind of "platonic" one, in which God and the Logos (since these are consubstantial) represent the fullness of being and goodness and everything else. God has grace and truth and life of himself, whereas we only have these things to the extent that God shares them with us. For example, the Logos is life, and this life is the light of all people; to the extent that we share in this life, which really amounts to living in the light of the Logos, then we have by participation and by grace what the Logos has by nature.
John's theology, in other words, is one of theosis. Those qualities which God has by his nature, he wishes to share with human creatures. In this way, they become participants in the divine nature (2 Pet 1:4), obviously while remaining creatures and not assuming a substantial union with the Creator, which is impossible. God wants to share his life with human creatures, so that they can enjoy the bliss and joy which he has in himself, through a union with and likeness to him.
This "fullness" about which John here writes really starts from the fullness of existence. God exists necessarily, having existence of himself, or rather (to follow Thomas Aquinas) he exists necessarily because he just is subsistent being itself (ipsum esse subsistens). Everything else exists only in a derivative and borrowed fashion, because God imparts existence upon everything whatsoever. Just as food is only hot because of the fire which heats it, whereas the fire has heat of itself, so also everything else exists only in a contingent matter because it is given existence by God who has it of himself.
There is also another fullness, however, which is shared with those who believe in Jesus Christ, who is the Logos of God. Christ says: The thief comes only to steal and kill and destroy. I came that they may have life, and have it abundantly (John 10:10). As I've commented in previous posts, there is a sort of life which is only possible for those who accept the teaching of the Logos and who are committed to it. This is the fullness of life, that powerful experience of joy at one's circumstances, which knows that one has been forgiven by God of one's sins and that one is living life in keeping with the truth. That sort of fullness is in principle impossible to have if a person doesn't believe in the Logos.
John says it like this earlier in the Prologue to his Gospel: But to all who received him, who believed in his name, he gave power to become children of God (1:12). Whereas the Logos is the Son by nature, and thus John calls him the only-begotten Son (v. 14), those who accept the Logos's teaching in faith receive the power to become children of God by grace. What the Logos has by nature, we can have by grace and by participation, because he is willing to share it with us.
Therefore John says that what we have received from him is grace upon grace. All these wonderful things which the Creator has of himself -- existence, life, light, etc. -- are shared with us, even though we are opposed to him, even though we hate him, even though we live in wickedness. Who can claim to have earned the Creator's offer of sonship? Nothing in principle could have merited the invitation to adoption into God's family, because our prior state is a sinful one in which atonement has to be made. But God, out of his goodness -- or better said, out of his grace -- is willing to do everything that is necessary for us to be saved and to enjoy the good things he wishes to share with us.
In times of difficulty, of stress, of pain, of hurtfulness, we would do well to reflect on the truth that we have all received from his fullness. If things in our circumstances hurt us, we turn to God and find a kindly face, willing to give us grace upon grace, if only we will accept it in faith.