What does Matthew 24:43 mean?
The return of Christ (Matthew 24:3) will be unexpected. The signs will be unmistakable (Matthew 24:27–31), but only once the critical moment has come. Jesus will return as the judge (Hebrews 9:28) without warning as everyone is going about their daily lives (Matthew 24:37–39). Since nobody can possibly know at what moment that will be (Matthew 24:36), Jesus insists His followers must be constantly vigilant and prepared.An analogy illustrates this point. One can compare Christ's return to a robber who breaks into houses (2 Peter 3:10). The thief's advantage is that he's never expected at a specific moment. If the homeowner knew exactly when the criminal would come, he could simply wait until then to be ready. Since the homeowner cannot know the time, he must stay awake at all times of the night.
The point is not that followers of Christ must literally avoid sleep out of fear of His return. The comparison is merely for the sake of that narrow point: they need to be ready because the exact moment in question will not be known. All people should live in a state of constant preparedness for the return of the judge. Even if Christ's return is many years away, it's common to meet an unexpected end to earthly life (James 4:13–14).
Those who read or hear Jesus' words should not delay in trusting Him for their salvation from sin (John 3:16–18; 2 Peter 3:3–7). Those who have already trusted Him should not carelessly lapse into sinful living as if the judge will not return at any moment.