The Seventh-day Adventists, just like the Mormons, try to (mis)use John 17 (especially verses 11 and 22) to teach their polytheistic "godhead"--that "God" is not actually one, but is "one" only as believers are. But they get it backwards--they try to create "God" in the image of man (or men plural!). John 17 cannot be used to say that God is "one" only in the way that believers are "one."
What Jesus seems to be saying in John 17 is that the "oneness" of believers imperfectly reflects the oneness of God. It cannot be turned around to say that it means that God is "one" only as believers are. Notice that Jesus does not say that He and the Father are one as believers are, but rather He talks about believers being one as He and the Father are.
In His prayer in John 17, Jesus draws several such parallels, but it is clear that what He is saying regarding believers is not to be in the same way as it is for Him. Here are some examples:
"They are not of the world, even as I am not of the world." (John 17:16 NASB.)
Here we see that both Jesus and believers are "not of the world"—but obviously not in the same way. Jesus is the eternal Creator of the world, and believers are not.
"As You sent Me into the world, I also have sent them into the world." (John 17:18 NASB.)
Again, Jesus was sent into the world, and His followers have been sent into the world—but obviously not in the same way. For one thing, His followers did not come to the earth and become incarnate, like He did.
"For their sakes I sanctify Myself, that they themselves also may be sanctified in truth." (John 17:19 NASB.)
Again, Jesus sanctifies Himself, and believers are sanctified--but there is a difference. Jesus says He is sanctifying Himself for His believers' sakes.
"The glory which You have given Me I have given to them, that they may be one, just as We are one;" (John 17:22 NASB.)
Of course, Jesus did not give His glory to the disciples in the same way. And, coming back to the original point, believers are not one in the same way that Christ and the Father are one. As the Bible teaches and the Nicene Creed affirms, Jesus Christ is "of one Being with the Father."
The Greek word in John 17:22 that is translated "just as" (or "even as" in the KJV) is the same word that is used in John 17:11, 14, 16, and 18. For more information on this Greek word, click here.