Here’s a brief “earliest” Church History look at it, to quote (from the ‘earliest’ church writings outside the Bible):

“In the early Church, Justin Martyr, a Church Father, taught that those who lived according to the logos are Christians, though they might not know about Jesus Christ.

Tertullian held that Christ has descended into Hades to deliver the Good News, with Clement of Alexandria, Origen and Athanasius declaring that “Jesus delivered from hell both Jews and Gentiles who accepted the gospel and that postmortem evangelism continues even today”.

Augustine of Hippo, however, believed that the unevangelized are condemned to hell and Thomas Aquainas held that those “brought up in the forest or among wolves” would be sent “the gospel message through miraculous means.”

Source: https://en.m.wikipedia.org/wiki/Fate_of_the_unlearned

Simple questions to ponder:

(i) Doesn’t Justin Martyr’s claim contradict both Catholic and Protestants view of it?

(ii) Doesn’t Tertulian’s claim contradict both Catholic and Protestants view of it too?

(iii) How come the current Catholic and Protestant view of it is similar to Augustine only in this aspect?

(iv) Can you please name one Christian theologian that taught the same as Augustine and existed before Augustine?

If you cannot find a theologian before Augustine preaching the same as he did in regards to this topic, doesn’t that contradict 2 Peter 1:20?

Please take note that Justin Martyr, Tertulian, Clement of Alexandria, Origen and Athanasius were all theologians “before” Augustine.

So, the truth regarding 1 Peter 3:18 – 19 is only learnt correctly via Augustine?

(v) Afterlife evangelism is clearly recorded in the Vision of the Shepherd of Hermas.

Is this a heretical book too? Who decides that? Holy Scripture mentions this Mystery clearly in 1 Peter 3:18 – 19 and later in 1 Peter 4:5 – 6. Perhaps, these are “secret” things of the kingdom of the heavens which is not known to all (Matthew 13:11).

To quote:

“One more example will illustrate the righteous spirits’ taking the gospel to the wicked spirits and will also provide a transition to the topic of vicarious work for the dead. The Shepherd of Hermas (first century) was, according to the fourth-century Church historian Eusebius, considered by some valuable for instruction in the Church and was quoted by some of the most ancient writers.

Hermas saw in a vision that the Apostles took the gospel into the spirit world so that the dead might receive the seal of baptism:

These apostles and teachers, who preached the name of the Son of God, having fallen asleep in the power and faith of the Son of God, preached also to those who had fallen asleep before them, and themselves gave to them the seal of the preaching [baptism]. They went down therefore with them into the water and came up again, but the latter went down alive and came up alive, while the former who had fallen asleep before, went down dead but came up alive. (Sim. 9.16.5)

Clement of Alexandria also cited this passage, commenting “that it was necessary for the apostles to be imitators of their Master on the other side as well as here, that they might convert the gentile dead as he did the Hebrew.”

Why was it kept a Secret?

To quote (from the same source again):

“The New Testament contains many references to mysteries of the kingdom that were shared only in the Savior’s most intimate circle.

Some of the early Fathers exhibit this same sense of secrecy about the special doctrines that the Savior taught, probably during the mysterious forty-day ministry (see Acts 1:3).

With regard to secrecy, Clement of Alexandria says of himself, that he writes “in a studied disorder” and has “here and there interspersed the dogmas which are the germs of true knowledge, so that the discovery of the sacred traditions may not be easy to any one of the uninitiated.”

Source:

https://rsc.byu.edu/archived/sperry-symposium-classics/visions-christ-spirit-world-and-dead-redeemed

(vi) Afterlife evangelism is also witnessed Sadhu Sundar Singh in his visions of heaven and hell agreeing to all this (e. g. ‘hindu saints’ accepting the Lord as their Saviour in the afterlife).

Is he a heretic too?

Proof of his visions from his ‘first and original’ biographies, please consider (images in links below, examples):

1) https://www.anonymouschristian.org/blog/secrets-beyond-hell-sadhu-sundar-singh/

2) https://www.anonymouschristian.org/blog/sadhu-sundar-singh-a-christian-universalist/

(vii) Wisdom from Ancient Judaism

Even Judaism has a concept of the Blessed Messiah Saving from the Last Judgment of Gehenna (Lake of Fire) itself agreeing to all this (again), to quote:

To quote (Page 34):

“Hanhart (1966:32) states that according to Jeremias, however, it is fundamental

for our understanding of the New Testament to make a sharp distinction between Hades

and Gehenna.

Hades is used exclusively for the interim period, Gehenna on the other side

for a state of punishment after the last judgment. In Hades, the punishment is provisional, in Gehenna eternal.”

To quote (Page 38):

“Bernard (1915:290) quotes two passages from the Bereshith Rabba to show that this idea was prominent in Jewish literature. He is, however, unsure of the dating of the literature.

Nevertheless, it still demonstrates the basic purpose of the descent:

‘When they are bound, they that are in Gehinnom, saw the light of the Messiah, they

rejoiced to receive him’; and ‘This is that which stands written, We shall rejoice and exult thee.

When?

When the captives climb out of hell, and the Shechinah at their head.’

The only difference here is that the captives are in Gehinnom and not Sheol.”

Source for both Quotes are from this phD Dissertation (link below, please read it). Indeed, the phD thesis I referred to may be found in link below:

https://repository.up.ac.za/bitstream/handle/2263/27130/dissertation.pdf?sequence=1

(viii) No one can cross the Chasm?

It is true that no one can cross the chasm (Luke 16:26) as Abraham mentioned it simply because Lord Jesus Christ hasn’t Died to Cross it yet. Isn’t it obvious that Christ Crossed the Chasm as mentioned so literally in 1 Peter 3:18 – 19?

Are you going to stand in front of His Great White Throne of Judgment and claim that you didn’t ‘literally believe that Christ crossed the Chasm and preached to the dead’ as 1 Peter 3:18 – 19 reveals? Are you going to tell Him that ‘you valiantly believed that He could not Save from Hell’?

Don’t you know that each time phrases such as ‘no one can do it’ or ‘all created things’ are referred to in Holy Scripture, it clearly EXCLUDES God Alone (the Creator, in Principle of 1 Corinthians 15:27) and that means that though ‘it may be impossible for a mere creation’, it is certainly POSSIBLE for GOD ALWAYS (example: Matthew 19:26 was answered in the Context of Salvation to the question ‘who then can be saved’?)

I’m pretty sure you and Augustine know the ‘limits’ of God and that Archangel Gabriel has ‘no clue’ when he said that NOTHING will be IMPOSSIBLE with GOD (in Luke 1:37) in the CONTEXT of GOD DOING HIS WILL which includes SAVING ALL MEN (e.g. 1 Timothy 2:4-KJV where believers are more special-1 Timothy 4:10 despite afterlife Judgment first in the Lake of Fire for some-Revelation 21:8 but Reconciliation toward these ‘thirsty ones from the Lake of Fire’ thereafter-Revelation 22:17).

Conclusion

Are all of them heretics and condemned to eternal Hell for believing this?

Please, be “Wise”.

“For in the way you judge, you will be judged; and by your standard of measure, it will be measured to you.” – Lord Jesus Christ (Matthew 7:2, NASB)

“Blessed are the merciful, for they shall receive mercy.” – Lord Jesus Christ (Matthew 5:7, NASB)

“For judgment will be merciless to one who has shown no mercy; MERCY TRIUMPHS OVER JUDGEMENT.” – apostle James (James 2:13, NASB)