Imamat 2

" And those who disbelieve say: why has not a sign been sent down upon from his lord? you are only a warner and (there is ) a guide for every one. "
Holy quran 13:7

This verse points that the earth has never been free from a guide and it will never be, there should be a Warner and the holy prophet is a Warner for the people in all times and the imam (guide) in his progeny is also for people in every age, prophet Muhammad PBUH said :

" I am the Warner and Ali is the guide, O Ali, through you those who are guided will receive guidance, "
from Fakhr razi and Syouti tafseer, narrated by Ibn Abbas

Imam Sadiq (PBUH) narrated a saying from Imam Sajaad (PBUH) that as of the creation of man kind by Allah, the earth has never been free from the guide, the Guidance has been sometimes explicit and the other times implicit, and until the resurrection , the earth won't be without righteous Guide(Imam), and Allah wont be worshiped without the Guide (Imam ),

therefore there are 3 different interpretations of this verse,
1- The holy prophet is a warner and a guide for every nation.
2- the holy prophet is a warner and every nation had a guide
3- the holy prophet is a Warner and every nation has a guide.

The last interpretation must be accepted according Aqa Mahdi Pouya and Almizan Tafseer by Alama Taba'taba'ee