I have some personal opinions for why the Lord did this.
When Cain murdered Abel, he denied his brother of any future progeny, which is one of the main reasons for why we enter into mortality.
The Lord allowed Cain to live and to have children so that the possibility would one day arise when some of Cain's descendants would be raised unto Abel.
Just like how in the Law of Moses when a man dies without having children, his brother would then marry his deceased brother's wife and have children by her that would be dedicated to the deceased brother.
I don't know exactly when or how this may be done, but it is my opinion that this was the reason for why Cain was spared.
I do too. I believe there is a lot more that was either lost to time or never recorded.
These are huge topics that I could never do proper justice.
I'm gonna share a link to a definition of faith and accompanying scripture references.
Hopefully this will help us narrow down the discussion. Get more specific and detailed questions and answers.
Faith in Jesus Christ
Salvation is basically being saved from the effects of both sin (spiritual death) and physical death brought on by the Fall of Man.
The Lord Jesus Christ, through His suffering, has already redeemed all people from the Fall, meaning that no one is blamed for it.
We all come into this world innocent and sinless although we still suffer the effects of the Fall such as temptation to sin, pain and eventual physical death.
Since the Lord Jesus Christ suffered death and conquered it through His Resurrection, all of Mankind will one day receive this free gift of Resurrection, which is salvation from physical death.
Our spirits will be reunited with our physical bodies, only this time our bodies will be perfect. No more pain, disease, weakness or death.
Being saved from sin, also called spiritual death (because being separated from God is death to our spirits) requires action on the part of the individual.
We can receive forgiveness of sin in this life by keeping God's commandments, which can cause us to live with greater feelings of peace, love and compassion thus receiving certain benefits of salvation in mortality.
In all honesty everyone in the world, regardless of any factor, is promised salvation eventually.
The word "eventually" is what I want to focus on.
The Lord Jesus Christ has suffered for the sins of all the world. Not just the educated. Not just the spiritual. For
all the world.
However,
when a person will receive the benefits of that salvation depends on the individual.
Those in mortality who live righteously, rely on the Lord through faith, repent of their sins and enter into the sacred covenant of baptism can enjoy many benefits of salvation
now in mortality.
They cannot be saved from physical death in this life, but they can overcome many sins and receive the blessings that come from that.
No one can overcome sin or be forgiven of sin without having a knowledge of the Lord Jesus Christ and what He has down for us.
Even if you procrastinate your repentance until you enter the spirit world after this life, you must learn about the Lord Jesus Christ in order to receive salvation from sin.
It is always better to repent of your sins ASAP because you can gain more wisdom and experience. The more you have when you depart this life the better off you will be in the next.
I understand how that can be confusing and it comes from a common misunderstanding.
The "spirit world" is still bound by time. It does
not exist in eternity. We do not enter eternity immediately upon death.
The spirit world is here upon the Earth and all the spirits of Mankind will go there immediately upon leaving their physical bodies.
We wait in the spirit world until the time of our Resurrection. Then after we are resurrected we receive our Final Judgmnt and
then we enter into eternity.
It is possible to continue to learn and grow in the spirit world. The more you learn and grow in mortality, the greater the learning and growing will be in the spirit world. Exponentially so.
This is why I stress the importance of coming to the Lord Jesus Christ ASAP so you can learn and grow as He would want you to.
Are you kidding? I'm having a blast!