I am working with John Ciardi's translation, New American Library, 1954.
There are seven stanzas before this one which give some context to the lines you're asking about.
No tortured wailing rose to greet us here
but sounds of sighing rose from every side,
sending a tremor through the timeless air,
a grief breathed out of untormented sadness,
the passive state of those who dwelled apart,
men, women, children — a dim and endless congress.
And the Master said to me: "You do not question
what souls these are that suffer here before you?
I wish you to know before you travel on
that these were sinless. And still their merits fail,
for they lacked Baptism's grace, which is the door
of the true faith you were born to. Their birth fell
before the age of the Christian mysteries,
and so they did not worship God's Trinity
in fullest duty. I am one of these.
For such defects are we lost, though spared the fire
and suffering Hell in one affliction only:
that without hope we live on in desire."
I thought how many worthy souls there were
suspended in that Limbo, and a weight
closed on my heart for what the noblest suffer.
The first group of people in Hell whom the poets see are these poor sad souls. They're not being tortured; they're just unhappy because they cannot ever achieve Heaven.
Virgil is pointing out to Dante "Hey, do you see these folks? Don't you care who they are? This is my spot in Hell, by the way. These people are sinless" [which is a very big deal in this text; sin is what people are being punished for] "but they still aren't good enough to get into Heaven. They were born before Christ, so they weren't baptized in the name of Jesus. That alone is enough to keep them out of Heaven. We want to go, but we aren't allowed, and that hopeless wanting is our torture."
The key stanzas in your question:
"Instruct me, Master and most noble Sir,"
I prayed him then, "better to understand
the perfect creed that conquers every error:
has any, by his own or another's merit,
gone ever from this place to blessedness?"
(emphasis mine) Creed is an entire belief system, not one person's individual faith.
My sense of that phrasing is that Dante is asking Virgil this:
"I've followed the Christian religion all these years, and I'm finally now seeing the end results of people who didn't follow it. Christianity is presented to us as perfect — it's supposed to be stronger than any sin, any mistake, any evil. But you and these other people are 'virtuous.' You would have gone on to Heaven if you had been born after the birth of Christ and had the chance to accept him as the son of God, the truth the way the life etc. It's not your fault that you weren't. That seems unfair — imperfect. So tell me,"
and this is where we reach the next lines:
"have any virtuous pagans gotten out of Hell and gone on to Heaven, either because they were just so virtuous or because God Said So?"
So if there were some pre-Christ believers who went on to Heaven, that would mean the creed/religion/system is ultimately still fair and therefore perfect. (He's asking for a rationalization of something he already believes.)
Virgil answers by listing a group of virtuous pre-Christians (since they weren't really "pagans") who were hand-picked by Christ to enter Heaven: Adam, Abel, Abel's son, Noah, Moses, Abraham, David, Israel, Rachel, "and many more He chose for elevation/among the elect."
Ciardi continues in the footnotes to this section to clarify:
"53. descended here: The legend of the Harrowing of Hell is Apocryphal. It is based on I Peter, iii, 19: "He went and preached unto the spirits in prison." The legend is that Christ in the glory of His resurrection descended into Limbo and took with Him to Heaven the first human souls to be saved. The event would, accordingly, have occurred in a.d. 33 or 34. Virgil died in 19 B.C.
