"In my sixth consulship, with Marcus Agrippa as my colleague, I made a census of the people. I performed the lustrum after an interval of forty-one years. In this lustration 4,063,000 Roman citizens were entered on the census roll. 

A second time, in the consulship of Gaius Censorinus and Gaius Asinius, I again performed the lustrum alone, with the consular imperium. In this lustrum 4,233,000 Roman citizens were entered on the census roll. 

A third time, with the consular imperium,  and with my son Tiberius Caesar as my colleague, I performed the lustrum in the consulship of Sextus Pompeius and Sextus Apuleius. In this lustrum 4,937,000 Roman citizens were entered on the census roll."

And yet how could He have been admitted into the synagogue----one so abruptly appearing, so unknown; one, of whom no one had as yet been apprised of His tribe, His nation, His family, and lastly His enrolment in the census of Augustus----that most faithful witness of the Lord's nativity, kept in the archives of Rome?

"But there is historical proof that at this very time a census had been taken in Judaea by Sentius Saturninus, which might have satisfied their inquiry respecting the family and descent of Christ."

Are these two references to a single census? We know that there were a number of censuses conducted under Augustus, but how many under Sentius Saturninus? 

Sentius Saturninus was governor of Syria (which seems to be what Tertullian means by Judea) in 9-6 BC, which is usually taken to be the one that Tertullian is referencing, since it fits with the assumed identity of King Herod (died 4 BC)  in the Gospel of Matthew, and fits in with the 9 BC general census of Augustus.

However, another Sentius Saturninus was Governor of Syria in 19-21 AD. Since Tertullian is talking about an event that happened after 30 AD, and then says "at this very time a census had been taken" is he likely to be referring to a census from 40 years before, or something a bit closer in time, say at 10 years? The language favours the latter, so maybe there was a census under this second Sentius Saturninus, which is a different one to the earlier one he mentions under Augustus.  

But with his statement, Tertullian seems to be contradicting the Gospel of Luke, which clearly says that the census was under Quirinius the governor of Syria. As a faithfull Christian, is it likely that Tertullian was unaware of what the Gospel said, or had decided that he knew better? The answer could be that the immediate predecessor of Quirinius as governor of Syria was a third Saturninus in 4-6 AD, and that he and Quirinius shared the set up of the census.

It should also be noted that until the deposement of Herod Archelaus in 6 AD, Judea was not under direct Roman management. Therefore a governor of Syria (as Saturninus in 9-6 BC was) would not have been authorised to take a census in Judea. It was only after 6 AD that such an administrative task could be conducted.

Josephus tells us in Book XVII, Ch 13 of his Antiquities;

"So Archelaus' country [Judea] was added to the province of Syria [6 AD]; and Cyrenius [Quirinius] who had been consul, was sent by Caesar to take account of the people's effects in Syria" [which only now included Judea].