Wow, Jim! It would take an entire Bible study to answer all your questions completely. However, I’ll try to state the main points quickly.
All focus throughout the entire Bible was and is Jesus Christ. In the Old Testament, before Jesus came to earth manifested as a human baby, humanity was instructed to look for a Messiah/Savior who would save them from eternal torture for all eternity. Through their faith in what God said about sending a Savior that would come to save them, they received God’s promise of eternal life. (Ex. Abel, Enoch, Noah, Abraham, and others in Hebrews 11.) It was their faith that was counted as righteousness, not their obedience or good behavior.
After Christ was born and grew up, for 3 years He told everyone He was God and that no one could come to the Father except through Him. He preached the gift of eternal life through faith in believing He was who He said He was.
After Christ suffered, died, and was resurrected, the apostles preached eternal life was offered to everyone, but would only be given to those who believed on the work of Jesus Christ on the cross ALONE. There was no work or deeds that would satisfy God’s wrath or eternal punishment except what His Son had already done.
So, there is no difference in believing on Christ from the 1st Century up to today. All belief/faith either looked TO the coming cross, or BACK at the cross for their eternal life. The bottom line is: eternal life is a gift offered to all men by God, but only those who rely on the suffering, death, and resurrecting of Jesus Christ alone as full payment for all their sins, (without adding any of their own good works), will be given eternal life. The last thing Jesus said before He died was, “It is finished”, because it was. All sins had been paid for. Once a person chooses to believe on Christ alone, he receives the free gift of eternal life promised to him by God.
Once a person knows without a doubt he’ll spend his eternal life in heaven, it impacts his life in all aspects. He can then experience the love of God through answered prayers, understanding His Word, and better appreciate His creation and people. (1 John 5:13 - These things I have written to you who believe in the name of the Son of God, so that you may KNOW that you HAVE eternal life.) When a person no longer fears death or hell, and when he knows God will always hear His prayers and help him in times of need, that person becomes liberated and free. Sure he’ll still make mistakes, but no longer does sin have to be his master. God promises He will never leave or forsake him, and God cannot lie.