When Sayid and Sun first see the four-toed statue, they are seeing the statue in a time when Jack and his accomplices in the 1977 crew tried to detonate Jughead, but are unsuccessful. Instead, Faraday's prediction about The Incident comes true, a large explosion happens, and the statue is sliced in half.
Sometime in the next few episodes, Locke is going to change something that causes the Incident to not happen. As a result, the statue never blows up. The people in the present day will suddenly see the statue as a whole. And from this, they know that something in the past has definitely changed.
Since whatever happened, happened, we know that time will try to correct itself in the present day. This means that the Incident, which didn't happen in 1977 (saving the lives of the 1977 crew) is destined to happen in the present day. So the final task of the people in the present is to get everyone off the island before something similar to the Incident happens again.
Sometime in the next few episodes, Locke is going to change something that causes the Incident to not happen. As a result, the statue never blows up. The people in the present day will suddenly see the statue as a whole. And from this, they know that something in the past has definitely changed.
Since whatever happened, happened, we know that time will try to correct itself in the present day. This means that the Incident, which didn't happen in 1977 (saving the lives of the 1977 crew) is destined to happen in the present day. So the final task of the people in the present is to get everyone off the island before something similar to the Incident happens again.
