"And this is life eternal, that we might know Thee the only true God, and Jesus Christ, whom Thou hast sent." In this verse is expressed the whole and entire purpose of the Incarnation, that we might know God--not as a mere philosophical concept or mental contruct, but personally. As our Lord said, "He who has seen me has seen the Father." Likewise, only he who knows Christ truly knows the Father.
For it is a true and certain saying that the fullness of the Godhead is revealed bodily in the Person of our Lord and Savior Jesus Christ, Who is the perfect image of the Father. This is the Gospel preached by the Holy Apostle Paul, the Gospel which "is not after man," but was made known "by the revelation of Jesus Christ."
This is why the veneration of icons upheld by the 7th Ecumenical Council is solemnly celebrated every year as the Sunday of Orthodoxy. Because God took on human flesh and assumed visible form through Jesus Christ, it is not only lawful but indeed essential that we venerate Him through the holy icons.
Indeed, "God is the Lord and has revealed Himself to us:" not in the indistinct shadows and images of the Old Testament, but face to face. As the holy Fathers say, "God became man that men might become gods"--through union with Christ, we become by grace everything that God is by nature.
This is what it means to know God: not by rumor or hearsay but first hand, through personal and intimate experience. This is eternal life and salvation and the reality proclaimed and safeguarded by the holy Fathers of the 7th Ecumenical Council. This is the true Orthodox Faith and the refutation of all heresies. This is the Faith that has established the universe.