What does Mark 13:35 mean?
Jesus' words here are mainly to Jewish and Gentile believers during the tribulation. Jesus has given them what they need to know to be able to roughly predict when He will return. Starting from the moment the Antichrist arranges a treaty between Israel and its enemies, they have seven years. When the Antichrist defiles the Jewish temple, they have three and a half (Daniel 9:27).Throughout those years, life on earth will get very hard. In addition to the catastrophic impact of God's wrath on the world, the Antichrist will persecute Christ-followers. They will not be able to buy or sell on the open market (Revelation 13:16–18). Countless believers will be killed for their faith (Revelation 13:7; 17:6).
The time periods match the way the Romans divide the night watches. "Evening" is 6 pm to 9 pm. "Midnight" is 9 pm to midnight. "When the rooster crows" is midnight to 3 am. Dawn is 3 am to 6 am. Neither Matthew nor Luke record Jesus being so specific.
This verse does not mean believers will lose their salvation if they are not paying attention and actively anticipating Jesus' return. Once we are saved, our salvation is assured for eternity (Ephesians 1:13–14). And salvation is never earned or kept by works (Ephesians 2:8–9).