Calvinist John Piper talks about John 9:

But that is what Jesus is saying here in verse 3: “It was not that this man sinned, or his parents.” In other words, this blindness—this specific suffering—is not owing to the specific sins of the parents or the man. Don’t look there for the explanation.

Then he tells them where to look. Look for an explanation of this blindness in the purposes of God. Verse 3: “It was not that this man sinned, or his parents, but that the works of God might be displayed in him.” The explanation of the blindness lies not in the past causes but the future purposes.

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One of the ways they try to escape the teaching of this text is to say that Jesus is pointing to the result of the blindness, not the purpose of the blindness. When Jesus says in verse 3, “It was not that this man sinned, or his parents, but that the works of God might be displayed in him,” he means, the result of the blindness is that God was able to use the blindness to show his work, not that he planned the blindness in order to show his work.

But there are at least three reasons why that won’t work.

1. One is that the disciples are asking for an explanation of the blindness, and Jesus’ answer is given as an explanation of the blindness. But if you say God had no purpose, no plan, no design in the blindness but simply finds the blindness later and uses it, that is not an explanation of the blindness. It doesn’t answer the disciples’ question. They want to know: Why is he blind? And Jesus really does give an answer. This is why he’s blind—there is purpose in it. There is a divine design. There’s a plan. God means for his work to be displayed in him.

2. Here’s another reason why that suggestion doesn’t work. God knows all things. He knows exactly what is happening in the moment of conception. When there is a defective chromosome or some genetic irregularity in the sperm that is about to fertilize an egg, God can simply say no. He commands the winds. He commands the waves. He commands the sperm and the genetic makeup of the egg. If God foresees and permits a conception that he knows will produce blindness, he has reasons for this permission. And those reasons are his purposes. His designs. His plans. God never has met a child from whom he had no plan. There are no accidents in God’s mind or hands.

3. And third, any attempt to deny God’s sovereign, wise, purposeful control over conception and birth has a head-on collision with Exodus 4:11 and Psalm 139:13. “The Lord said to Moses, ‘Who has made man’s mouth? Who makes him mute, or deaf, or seeing, or blind? Is it not I, the Lord?’” “You formed my inward parts; you knitted me together in my mother’s womb.”