It seems clear that the words "lift up" refer to Jesus' crucifixion.
John 12:32-33"And I, if I am lifted up from the earth, will draw all men to Myself." 33 But He was saying this to indicate the kind of death by which He was to die.
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NOTE: There is a popular song that states: "If I be lifted up, I will draw all men unto Me." These lyrics interpret "lifting Jesus up" as exalting and worshipping Him." The meaning of this song standing alone is wonderful, but are not an accurate interpretation of that verse.