Christianity is the belief in one eternal self-existent God. God as the creator of the world, placed the first two people, Adam and Eve in a garden where He walked and talked with them. He gave them only one rule to not eat the fruit of a certain tree.
A broken relationship
When they disobeyed God’s instruction not to eat the tree, sin and death entered the human race and the relationship with God was broken. Sin is the wrong things we all do which separate us from a Holy and Good God.
God had a plan
God set in motion a plan to save (redeem) us. He gave us the Bible (scriptures) to tell us about himself and about the plan of salvation so we can be reconciled to Him. The Bible tells us that while there is one true God, we can know Him as God the Father, God the Son, (Jesus Christ) and God the Holy Spirit.
Jesus Christ, the Son came 2000 years ago and lived as one of us. Although perfect, He was rejected, He stood in our place, He took our sin and our punishment through being crucified by men on a wooden cross. Jesus was resurrected back to life and through faith in Him, our relationship with God is fully restored. Ephesians 2:8-9
A restored relationship
While Jesus is currently in heaven, we await His return to finally put an end to death and the troubles of life and to restore the earth to its original design. In the meantime, God, the Holy Spirit is with us in the hearts and lives of believers. This restored relationship enables us to call God, our Father, a good and wonderful loving Father. We become part of God’s family. 1 John 3:1
Reading through this website will help you understand these things.