"This is eternal life: that they may know You, the only true God, and the One You have sent—Jesus Christ." John 17:3 (HCSB)
Firstly, Jesus did not say, "Eternal life is that they may know the only true God. Period."
What He did say was (italics mine),
Jesus said eternal life is found only by those who know the Father and the Son.
Secondly, what Jesus is making here is a direct claim to deity. Previously in the Gospel of John, Jesus had already made His teaching plain: He and the Father are one, and not just in purpose either. Jesus says again and again in the Gospel that He and the Father are equals. That is why the Jews sought to kill Him. And Jesus did not just say He was equal with God, Jesus also used the Divine Name YHWH God for Himself.
And thirdly, another very important word in the verse is the word, "know." As used in other passages in the New Testament, the word denotes a deep relationship, an intimate connection with the object of that knowledge. In the same way that love has always existed in the Godhead, the invitation is open and extended to all. Jesus repeats the point elsewhere in the NT.
"Listen! I stand at the door and knock. If anyone hears My voice and opens the door, I will come in to him and have dinner with him, and he with Me."
It's all about a true and fully committed relationship. You and God, one-on-one. And the good news is all you need to do to start your relationship with God is to ask Jesus.