There is evidence that Greek continued to be used in Massalia (Roman name: Massilia) even during the late empire and the spread of Christianity. To cite one academic source,
...the city preserved its Greek language until the 5th c. A.D., and
remained an active cultural and commercial center.
Source: Richard Stillwell (auth.), William L. MacDonald and Marian
Holland McAllister (eds.), 'The Princeton Encyclopedia of Classical Sites' (1976)
The same assertion is made in this 1981 doctoral thesis. It is worth emphasizing at this point that, even from very early times, Massalia was a multilingual city with (probably) most inhabitants conversant in more than one language. Initially, this meant Greek and Gaulish as Greek colonists married local women. Later, Latin was added to the mix.
Circumstances brought three languages into use at Massalia, the Greek,
the Latin, and the Gallic (Isid. xv., on the authority of Varro). The
studies of the youth at Massalia in the Roman period were both Greek
Roman culture and language did replace Greek in some respects but mostly it existed alongside it. This should not come as surprise given that for elite Romans, Greek was a language most of them studied and acquired, along with Greek thought.
Greek had held an important place in Roman society and culture since
the late Republican period, and educated Romans were expected to be
bilingual and well versed in both Greek and Latin literature.
Source: Alison John, 'Learning Greek in Late Antique Gaul'
Romanization and Hellenization are not distinct phenomena: being Roman
involved, to a certain extent, being Greek (§1.2). In many parts of
the Western Empire, we are in fact often faced with a ‘cultural
triangulation’, a series of negotiations of identities which involved
Roman, Greek and local.
Source: Alex Mullen, Chap. 9 in 'Southern Gaul and the Mediterranean' (2013)
Our evidence for the continued importance of Greek during the period of the Roman empire comes from both authors and inscriptions. Strabo (d. circa AD 24) , who wrote his Geographica during the reigns of Augustus and Tiberius, observed that in Massalia
...all the men of culture turn to the art of speaking and the study of
philosophy; so that the city, although a short time ago it was given
over as merely a training-school for the barbarians and was schooling
the Galatae to be fond enough of the Greeks to write even their
contracts in Greek, at the present time has attracted also the most
notable of the Romans, if eager for knowledge, to go to school there
instead of making their foreign sojourn at Athens.
Strabo may have overstated the importance of Massalia as a centre for education as we have few surviving texts from Massaliote writers, but it was nonetheless of at least minor importance, and definitely Greek in outlook. Tacitus (d. AD 120), who had family roots in Massalia and probably spoke Greek (he served in Greek-speaking Asia), wrote about Agricola's education in the city.
We also have inscriptions in Greek dating to the 1st, 2nd and 3rd centuries AD though, because relatively little excavation work has been done, the sample is small. One example is:
The domain of education is represented by a white marble dedication
from Marseille dating to the end of the first or beginning of the
second century AD and dedicated to a .... ‘teacher’...
Source: Mullen, Chap. 9
Another example is a little later:
Again from Marseille, and dated to the end of the second, or beginning
of the third century AD, a marble plaque refers to what appears to be
a bilingual grammaticus.... The name and inscription are Greek...
Source: Mullen, Chap. 9
Interestingly, inscriptions (and, to a lesser extent, Roman authors) point to the use of Greek in several regions of Transalpine Gaul during the imperial period. Thus, we should not be surprised that
Hellenism was not a dying remnant of a culture which was disappearing
in favour of Romanization but a conscious choice on the part of the
community and its elite, and its cultural traditions were a dynamic
and evolving response to the changing context of the Roman empire.
Source: K. Lomas, 'Hellenism, Romanization and cultural identity in Massalia'. In Lomas (ed.), 'Greek Identity in the Western Mediterranean' (2004)
Overall, the epigraphic evidence is patchy and a little confusing with both Latin and Greek names and writing in evidence, but with the former seemingly more prominent. This might indicate more Latin than Greek speakers, but we have to consider survivorship bias and we cannot assume that the proportion of inscriptions for each language closely correlates with population.
Greek gods, cults and festivals were still in evidence in 3rd century AD; it seems reasonable to presume that ceremonies were conducted in Greek. In the 4th century AD, the spread of Christianity helped to maintain the use of Greek. Communities had sprung up in Massalia by the early 4th century and
Christianity was...a context in which Marseille's enduring Greek
cultural and intellectual tradition (still acknowledged in many ways
under the Empire) could be revived, its links with the interior
fostered, and its special relationship with the East reinvigorated.
A footnote to the above adds:
The city's Greek heritage is reflected under the Empire in the names
of its buildings, officials and individuals.... Greek was still used in
the early third century to record the career of a high-ranking
imperial functionary from Marseille...
Source: S. T. Loseby, 'Marseille: A Late Antique Success Story?'. In The Journal of Roman Studies, Vol. 82 (1992).
However, by the 5th century AD Greek was in decline among Gallo Romans though we cannot assume that it disappeared entirely. One of the last prominent Greek-speakers of the city that we know of was Gennadius of Massilia, a priest and scholar of the late 5th century. By the mid to late 6th century, the Greek historian poet Agathias (d. after 580 AD) wrote that Massalia, although
once a Greek city, it has now become barbarian in character having
abandoned its ancestral constitution and embraced the ways of its
Translation cited in Dallas DeForest, 'Agathias on Italy, Italians and the Gothic War'. In Estudios Bizantinos 8 (2020)