Jesus said, "And this is life eternal, that they might know Thee the only true God, and Jesus Christ, whom Thou hast sent." Take note: He doesn't say we must merely believe in God, or know things about Him ( such as we might learn from a theology textbook). He says, rather, that we must know Him.
Biblically, when it is said that a man "knows" a woman, what is meant is that the two have intimate relations together. It is the union of two souls and bodies, wherein "the two become one flesh." Likewise, to know God requires an actual union/communion with Him. And because God is at the same time eternal and the Source of Life, to know Him is life eternal.
Of course, it is impossible to enter into communion with God unless we struggle against the sinful passions while striving to fulfill His commandments, and this is possible only when we are strengthened by the grace of God--and the fullness of grace is only available within the one true Church that our Lord established on the day of Pentecost.
Christ prays further for His disciples "that they may be one, as We are," and herein lies the foundation for the unity and ultimate authority of the Church and the refutation of the Latin heresy of papism. The Church is conciliar. The bishops at an ecumenical council are empowered to proclaim the truth unanimously according to what seemed good to us and the Holy Spirit, as the Apostles did at the Apostolic Council in Jerusalem. Thus, the Fathers of the First Ecumenical Council justly condemned the heresy of Arius and cast him from the Church.
This is precisely because Arius taught that Christ is a creature, and thus He could not truly be one with the Father. Nor could we the faithful enter into eternal life through union with a creature who did not possess the fullness of Divinity. Christ ascended that He might--on the day of Pentecost--send down upon us the life-giving Spirit, through Whom we are all truly united to Christ. May we all strive to purify our hearts of every egotistical thought and desire, that we may be proved worthy of this Gift.