I think a proper answer to this question requires that we (re-)examine many of the verses used to form the opinion "Faith in Jesus is necessary for salvation."
For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.
What it says: Anyone who believes in God's son will not perish.
What it doesn't say: Only those who believe will not perish.
Jesus answered, “I am the way and the truth and the life. No one comes to the Father except through me.“
What it says: It is impossible to get to the Father, except by way of Jesus.
What it doesn't say: That understanding this, or knowing the name of Jesus is necessary to get to the Father.
For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.
What it says: The gift of eternal life is made possible by/through Jesus.
What it doesn't say: Again, it doesn't say that understanding this is necessary to receive eternal life.
Whoever believes in the Son has eternal life, but whoever rejects the Son will not see life, for God’s wrath remains on them.
What it says: Whoever believes in God's son has eternal life; Whoever rejects God's son will not "see life."
What it doesn't say: It says nothing about those who have neither believed in God's son, nor rejected His son.
He answered, “‘Love the Lord your God with all your heart and with all your soul and with all your strength and with all your mind’; and, ‘Love your neighbor as yourself.’”
What it says: Loving God and our neighbors is necessary to inherit eternal life.
What it doesn't say: Knowing the name of Christ, or understanding his work of salvation on the cross is necessary to inherit eternal life.
And in the dialog between Jesus and Peter, Jesus repeats the phrase "Feed my lambs," but never once talks about a requirement for understanding salvation.
So what I believe we're left with here is two solid truths:
- Those who believe in Christ (which requires knowing of him) are saved.
- Those who reject Christ (which also requires knowledge of him) are not saved.
There is a big gap between these, though. To help fill that gap, we can consider that we know many people were saved before Christ's coming. (see this question).
From this, we see that the requirement for salvation is not actually knowledge of Christ, but rather a faith in God to save us from ourselves. Prior to Christ's coming, nobody knew the mechanism of salvation; they only knew that God could save them, and their faith in Him to save them is what counted.
Adam and Eve didn't even know the term "messiah," they just knew of a (to them) vague promise for retribution against sin (see Genesis 3:15).
If Adam and Even can have faith in this promise, without knowing of Jesus, then surely anyone can.
We are even told in Ecclesiastes 3:11 that God has revealed Himself to all humanity:
He has made everything beautiful in its time. He has also set eternity in the human heart; yet no one can fathom what God has done from beginning to end.
A silly analogy
When I call my grandmother, who lives in Montana, I pick up the telephone, I dial her 10-digit phone number, I listen to a few ringing sounds, then I hear "Hello?"
I've worked in IT and telecommunications long enough that I understand the basics of how touch-tone dialing works, telephone switches, and even how microphones and speakers work in my handset and hers. One could easily make a statement such as:
The telephone switch at the CO in Denver is the way to call your grandmother. Nobody makes telephone calls to Montana, except through this switch.
... or ...
Whoever believes in the telephone switch will be able to make phone calls; but whoever rejects the telephone switch will be denied the ability to make phone calls.
I fall into the category of people who understand these telephone switches, and believe in them.
There is a far greater number of people, though, who have never even thought about a telephone switch. But they still pick up the telephone to call their grandmother, having faith that the call will go through.
Is knowledge of the telephone switch necessary? No. Is faith in the telephone switch necessary to make a phone call? Yes.
What is the evidence of this faith? People picking up the phone and dialing.
What is the evidence of faith in God? "Feed my sheep."
Any person can respond to the revelation of God they have experienced, and understand that they are imperfect and sinful, and that only God's forgiveness is sufficient for them to be "right with God."
I don't believe a knowledge of Christ, or understanding of His sacrifice, or even an understanding of the concept of "Heaven" or "eternal life" is necessary for someone to be saved. What is necessary is that a person recognize that they are powerless on their own to be right with God, and that only through God's gift of compassion and grace, can they experience that right communion with God.