What does John 7:17 mean?

The phrasing used here by Jesus is unmistakable; the Greek word order corresponds to, "If anyone is willing the will His [God's] to do, he will know. Satan tempts man with limited knowledge based in disobedience (Genesis 3:5). But we are meant to obtain true knowledge through obedience. Jesus turns His era's assumed relationship between knowledge and morality backwards. Ancient philosophers frequently held that morality was something produced by knowledge. Under that assumption, the ability to do "good" was based on a person's understanding of moral and philosophical truths. Only those who could understand could obey, they thought. In other words, misunderstanding causes disobedience, per ancient philosophy.

According to Christ, disobedience causes misunderstanding. Rather than knowledge of the truth leading to obedience, Jesus claims that whether a person is willing to obey God is what affects their ability to learn the truth!

This general idea is echoed elsewhere in Scripture, both by Jesus and others (John 18:37; Romans 1:18–20; Hebrews 11:6). In fact, Jesus laid the groundwork for this idea when preaching in Capernaum, after feeding the thousands (John 6:29). The fact that Jesus was noted to be sinless (Hebrews 4:15), even by many of His own critics (John 8:46), demonstrates how a person's spiritual life says a great deal about their knowledge (or ignorance) of God's Word.