This leads to another key aspect to understand, when it comes to the Gospel. God is holy and perfect, but we are born as sinners. It is our sin that prevents us from being acceptable to God. God will not tolerate sin, and nothing that is sinful is allowed into Heaven; God's perfect place. So, in order for any of us to become friends with God, and be able to get into Heaven, our sin has to be dealt with.
Many people are familiar with Adam and Eve, and how mankind, first sinned, in the Garden of Eden. If you read the Bible, it's impossible to miss, since the account is described in Genesis, the first book of the Bible. Right after the description of how God created the universe, we learn about Adam and Eve sinning against God. It is this first act of sin, that caused mankind to become unfit to live with God in Heaven. This is explained in Genesis 3:22-24.
From a human perspective, it may not make sense, or even seem fair, that you, I, and all of mankind, are prevented from going to Heaven because Adam and Eve screwed up. However, just because we don't understand it, that doesn't make God wrong. What it does is reveal our limited level of understanding.
Now, I recognize that, for some, this may seem like a copout. We humans, often convince ourselves that we're able to understand anything, as long as we study it enough. And then we learn of things like paradoxes.
A long time ago, I wrote an extremely long, seemingly aimless, article, entitled Religion: The Forbidden Topic. I'll admit that it's all over the place, in terms of subject content. However, it makes some good points, concerning mankind's unfit status to live in God's home. Because of God's holiness, and because he's beyond our understanding, there will always be things that we don't understand. However, one foundational aspect of the Gospel, is understanding that our sin separates us from God.
It's important to point out that people don't get to Heaven by doing good deeds or by going to church. God's standards are so perfect that, if a person sins, even once, they're unfit to go to Heaven, and would, therfore, end up in Hell. For this reason, no one can rely on their good deeds to make them acceptable to God. Instead, we have to rely on God to fix this problem.
Fortunately, for us, God loves the human race, and wants everyone to go to Heaven. Without exception, he wants to be friends with all people. However, we have to trust his way of provision. It will happen by whatever means he chooses.
After Adam and Eve were thrown out of the Garden of Eden, generations passed before God began to reveal his plan of how he would fix the problem of sin. Although, this doesn't mean that he didn't have a plan from the very beginning.
Abraham was a godly man. God made a promise to him. He promised to bless Abraham, by giving him many descendants. God said that Abraham's descendants would become a great nation, and that all nations would be blessed through Abraham. Here's what the Bible says, in Genesis 12:1-3:
"The Lord said to Abram, 'Leave your country, your relatives, and your father's home, and go to a land that I am going to show you. I will give you many descendants, and they will become a great nation. I will bless you and make your name famous, so that you will be a blessing. I will bless those who bless you, but I will curse those who curse you. And through you I will bless all the nations."
-Genesis 12:1-3
So, going back to the first book of the Bible, God had a plan, and made a promise, to make things right between himself and mankind. That plan was revealed, gradually, throught many generations. He spoke to prophets, who wrote down prophesies about the future. Abraham's descendants, became the nation of Israel. God blessed that nation, from its very beginnings, as a promise to Abraham. God spoke to Moses, another godly man who was born as one of Abraham's descendants. By speaking to Moses, God gave the people of Israel, all sorts of rules and practices that they must follow, in order to obey him. This included things like animal sacrifices, which were used as signs of how he would, ultimately, pardon the sins of mankind.
He spoke to prophets, telling them that he would send a perfect servant. For thousands of years, and for many generations, the people of Israel knew to look for this coming messiah. Although, they never completey understood exactly how all events would unfold, or exactly what would happen, or when. Like Abraham, they just knew to trust God, with his promises.
Jesus was born, and was shown to be that promised messiah that the prophets had been speaking about. His birth was miraculous, since he was born of a virgin; as a descendent of Abraham. He performed a huge number of miracles, in the presence of large crowds, in order to prove that he is God's promised messiah, and, also, the Son of God. He lived on this earth, not only performing miracles, but explaining his role in how he was to make things right between mankind and God. He would be the means by which people would be able to be acceptable to God, and be able to go to Heaven, and avoid judgment, due to our sins.
The book of Hebrews does an excellent job explaining all of this; particularly chapters nine and ten. It explains that the animal sacrifices, which were prescribed by God in the early days of Israel, were a picture of the ultimate sacrifice that Jesus, himself, would become, when he was, eventually, tortured, and murdered, by evil men.
For about three and a half years, starting around the age of thirty, Jesus performed all sorts of miracles, and preached to crowds, telling them how to get into Heaven. Many people were confused, until after he was murdered by religious leaders. Before being killed, Jesus told his followers that he would be killed, but that he would come back to life, three days after the fact. This would be a sign that he had conqured death, and had completed the task of making it possible for ordinary people to, finally, be able to become right with God, and to be able to live with God, forever, in Heaven.
The teaching that Jesus gave us, is very clear, today. He taught that it's impossible for humans to be acceptable to God by any good works. The only way that we can become acceptable to God, and to become worthy of Heaven, is by putting our trust in Jesus to make us acceptable. Once a person does that, and chooses to follow Christ, by accepting him as their Lord and savior, they become acceptable to God, and are promised a home in Heaven, once this life comes to an end.
Making this choice, has been explained by using different terms. It's been called "being saved", "being born again", "becoming a Christian", or "becoming a child of God". It's a choice that a person makes, when they recognize that they are a sinner, and that they can only be made acceptable to God by putting their trust in his son, Jesus Christ. It's a lifelong choice to trust Jesus, and to start living the way that he says we need to live.
Godly people, like Abraham, had a similar trust, when he chose to trust God, and the future promise that God made to him. So is the trust that Christians have, when we choose to trust that Jesus is the means by which our sins are forgiven. Jesus is the means, by which, we are made acceptable to God.