What does John 11:9 mean?

Verses 9 and 10 contain Jesus' cryptic remark to the disciples, after the confusion over His intent to go back into Judea. It has been two days since Jesus received a message saying His friend, Lazarus, was sick (John 11:1–3). Lazarus lives in Bethany, very close to Jerusalem and the hostile religious leaders who want Jesus dead (John 5:18; 10:39; 11:8). When Jesus sends back a comforting response (John 11:4) and waits two more days, the disciples would have assumed that was the end of the issue (John 11:6). To their surprise, Jesus states His intent to head back to see Mary, Martha, and their now-dead brother (John 11:7). Naturally, the disciples protest by pointing out how dangerous this is.

Jesus' answer in these verses hinges on the idea of walking according to the will of God. It's not unusual for God's commands to be perplexing. Abraham was commanded to sacrifice his own son (Genesis 22:1–2). Naaman was commanded to wash his leprous body in a muddy creek (2 Kings 5:1–12). Jesus has just told two sisters that their brother's illness will not result in death—and their brother had already died. But Abraham saw God's eventual plan (Genesis 22:11–14), Naaman was healed (2 Kings 5:13–14), and Lazarus will soon be resurrected (John 11:43–44).

In the same way, following God's will can seem dangerous, from an earthly perspective. To be sure, there are times when being a follower of Christ means losing one's life for the sake of faith (John 16:1–2). But when God has other plans, then we can trust that His guidance will result in good outcomes. In this case, Jesus is pointing out that it's important to act while there is still daylight. That light prevents danger which comes after dark. Since Jesus is "walking in the light"—according to the knowledge and will of God—His actions are only as dangerous as God intends them to be.