God created man and in the beginning the relationship between God and man was very good. But man chose to disobey God. Man's disobedience made fellowship with God impossible:

For your depravity has separated you from your God;
and your sins have hidden His face from hearing you. (Isaiah 59:2)

The Bible teaches how the relationship between God and man can be restored. In order to understand this, we must first understand some things about God.

God is holy.

God is absolutely sinless and perfect in His character.

There is none holy as the LORD. (1 Samuel 2:2)
Holy, holy, holy, is the LORD of hosts. The whole earth is full of His glory. (Isaiah 6:3)
The LORD is in His holy temple. The LORD's throne is in heaven. ... His soul hates the wicked and the one that loves violence. (Psalm 11:4)

God is just.

Because God is holy, He cannot tolerate sin. Because God is just, He cannot allow sin to go unpunished.

The LORD...will by no means clear the guilty. (Exodux 34:7)
Though hands join together, the wicked will not go unpunished. (Proverbs 11:21)
[God] has established a Day in which He will judge the world in righteousness... (Acts 17:31)

Holiness and justice are two important characteristics of God.
But what are some characteristics of man?

Man is sinful.

Sin is the breaking of God's law, that is, the disobeying of His commandments in thought, word, or deed (1 John 3:4). Sin is a great offense to God; it is rebellion against Him.
The Bible declares, "all have sinned and fall short of the glory of God (Romans 3:23); for there is not a righteous man upon the earth who does good and does not sin. (Ecclesiastes 7:20)"

The separation between God and man is such that man's best efforts cannot satisfy God's holiness and justice. All his righteous acts are like filthy rags. (Isaiah 64:6)

Man is under God's wrath and must pay sin's penalty.

God is pure, and will not allow any sin into Heaven. "And there shall not enter into it [Heaven] anyone that defiles, or whoever makes an abomination or a falsehood...(Revelation 21:27)
God's wrath against sin is severe and hangs over the heads of all who reject His Word. God will cast sinners into Hell, where they shall be "tormented day and night forever and ever...in the Lake of Fire. (Revelation 20:10)
But we are not left without hope. God, by His love and mercy, has provided a way of escape.

How can the relationship between God and man be restored?

God is loving and merciful.

The LORD, the LORD God, compassionate and gracious, longsuffering in wrath, and abounding in goodness and truth, keeping mercy for thousands [of generations], forgiving iniquity, transgression, and sin ... (Exodus 34:6,7)
God's love prompted Him to provide a way for man's relationship with Him to be restored.
For God so loved the world that he gave His only begotten Son, in order that whoever believes in Him should not perish, but have eternal life. (John 3:16)

Jesus died as a substitute for sinful man.

Man cannot appease God's wrath through any of his own efforts. But Jesus Christ, the Son of God, came and livied as a man. He fulfilled God's law perfectly, and then offered Himself as a sacrifice in order to pay the penalty for man's sin.
Christ suffered for us... [and] bore our sings in His own body on the tree. (1 Peter 2:24)
God sent forth [Jesus Christ] as a sacrifice which appeases God's wrath, through faith in His blood. (Romans 3:25)
God's forgiveness is accessed by way of Christ's sacrifice.

Man must respond by believing.

Christ's sacrifice and God's forgiveness are not granted to everyone, but only to God's children. The gift of adoption into God's family is only offered through faith in Christ.
But to as many as...believed on His (Jesus') name, He granted the authority to become sons of God. (John 1:12)
Through faith, God's children are counted as "not guilty" and obtain forgiveness because of Christ's sacrifice. Through faith they also obtain the very righteousness of God because of Christ's merits.

Man must respond by repenting.

Adoption, forgiveness, and righteousness are freely accessed by faith alone, but not a faith which is alone. Faith is always evidenced by a change of life.
For just as the body apart from the spirit is dead, so also faith apart from works is dead. (James 2:26)
A person who claims to believe in Christ but continues to walk in darkness has not yet experienced true saving faith; he is not a Christian.
He that says, "I know Him," and does not keep His commandments, is a liar, and the truth is not in him. (1 John 2:4)

God:
   is holy, just, loving, merciful
Christ:
   died and received punishment in man's place
Man:
   must believe and repent to be saved

When you respond by turning from your sin and trusting Christ as Lord and Savior, you have the promises of evernal life, complete forgiveness and a restored relationship with God. (Romans 5:1, 8:1)

In prayer tell God...

1. That you recognize your sinful condition before a holy and just God, and that you are in need of Christ's forgiveness.
2. That you repent of your sins. Repentance involves the acknowledgement of past sins and the turning away from sin in order to follow Christ.
3. That you are basing your salvation on Christ alone, not on your own merits or good works.
4. That you are thankful to God for enabling you to:
   (a) understand the gospel,
   (b) submit to Christ as your Lord and your Savior, and
   (c) begin an intimate relationship with Him.

Enter through the narrow gate. For the gate is wide and the way is easy which leads to destruction, and many are they who enter through it. For the gat is narrow and the way is difficult which leads to life, and few are they who find it. (Matthew 7:13,14)
But these things are written that you may believe that Jesus is the Christ, the Son of God; and that by believing you might have life through his name. (John 20:31)