What does Luke 13:26 mean?

Jesus is explaining what it takes to be saved. He's addressing how only those who "know" Him can spend eternity with Him, but He has different standards for that knowledge than the culture does. Here, He mentions two typical ways the culture would define a deep personal knowledge.

First, they ate with Him. Specifically, they ate the food He provided (Luke 9:10–17). Hospitality and shared meals are still an important part of Middle Eastern culture. To eat with someone is to fellowship with them. To offer food, as Jesus did, is to offer protection. The problem is, they misinterpreted Jesus' motivation for feeding them. It wasn't a sign of deep permanent fellowship; it was a sign of Jesus' compassion towards their physical hunger. It was also a metaphor for His ability to sate their spiritual hunger. The people appreciated the food so much they wanted to make Him king (John 6:1–15). Jesus had to explain that they focused on their physical state when they needed to come to Him for spiritual life. Literal bread can bring life for a limited time, but they need the bread of life: Jesus, Himself, who came down from heaven and provides eternal life (John 6:35–51).

Second, the people had listened to Jesus teach in their streets. Surely, they thought, His presence in their neighborhoods was a sign that He identified with them. That isn't the case, however. Jesus preached to tens or hundreds of thousands of people. Very few took the time to understand what He taught. That is the point of parables: that those who don't really want to know the truth don't have to (Luke 8:10).

A similar teaching in Matthew points out another reason people might claim to know Jesus without actually knowing Him. Some self-labeled believers will perform great deeds in Jesus' name (Matthew 7:22). But even doing good works out of respect for Jesus does not identify a true follower. Jewish mystics learned this in Ephesus (Acts 19:13–16). Paul will later explain, "whatever does not proceed from faith is sin" (Romans 14:23). Saying you do works for Jesus isn't the same thing as knowing Him and being empowered by Him (Matthew 7:23; Luke 13:27).

These issues aren't unique to the crowds that follow Jesus. The same happens today. You can be in fellowship with other believers, pray, read the Bible, listen to sermons, even serve, and still not know Jesus. Doing such things should be markers of Christians, but they don't save. Only knowing Jesus—knowing He is the Son of God who came to take the sins of the world and asking Him to take your sins—ensures you can fit through the narrow door.