So, the first 400 years would cover, say, to 433 AD’ish? We can call it 450 AD for a nice round number.
Visually, you have things like the
Catacombs of St. Priscilla (2nd-5th c.), where you have some of the earliest surviving Marian imagery--
Virgin & Child (with Isaiah?); an
Epiphany; an
Annunciation.
Architecturally, you have things like the
Santa Maria Antiqua, which was built in the mid-5th c and is the earliest surviving Christian monument within the Roman forum. (Remember that the Edict of Milan, which de-criminalized Christianity [and other religions] only dated to 313.) Likewise, the Church of Mary in Ephesus dates to the early 5th c (possibly built in anticipation of the Council of Ephesus in 431, which solidified the concept of the Theotokos, which was first documented to have been used by Origen, who died c. 254). And there are other Roman churches, like the
Basilica of Santa Maria in Trastevere (floor plan and wall structure date to the 340’s) and the
Basilica di Santa Maria Maggiore (built sometime between 432-440).
Liturgically, I’m seeing references to her being included in Eucharistic prayers as early as 225 A.D., although I’m not seeing the exact wording written down.
Privately, there’s the prayer that starts off “Sub tuum praesidium Sancta Dei Genetrix”, which has been dated to a
3rd c. Egyptian papyrus called the Rylands Papyri.
You have Epiphanus writing c. 400, “Let Mary be held in honor. Let the Father, Son and Holy Ghost be adored, but let no one adore Mary”, which is pretty clearly saying, “Whoa, people, remember who Mary is pointing the way to-- she’s not an end unto herself.” Which is showing that she’s popular enough to require a warning. And similarly,
Ambrose writes around 381, “Mary is the temple of God and not the God of the temple”. Ambrose is also the one who first documented the Mater Ecclesiae title.