Why do many Christians talk about "believing in Jesus" rather than "believing in God"?

Good Question Turell. It actually goes to the heart of Christianity.

The earth was at the time of Jesus (and many times since then) dark with misapprehension of God. Because God is a God of justice and terrible majesty, Satan purports him to be severe and unforgiving. Jesus came also to clear away this belief. In looking at Jesus, we see an exact representation of God. The behaviour of Jesus and God is identical. If Jesus would stop to bless the children, God would do the same. To seek out the lost lambs is the focus of Jesus, and of God. Jesus words were exactly to this effect in saying "If you have seen me you have seen the Father."

Further, in the same section (John 14) Jesus declares that he is the way, the truth and the life. No one comes to the Father except through him.

All these are the reasons we focus on Jesus. Because he is the one we need to believe in, as given in John 3:16 where Jesus clarifies to Nicodemus that it is only through belief in him that we attain eternal life. Hence the focus on Jesus, he "in whom dwells the fullness of the Godhead bodily."

You do say however something else that needs clarification, in that Jesus was a temporary manifestation.

This is not the case. Jesus became a human, and remains a human today. A human shares the throne of God. Through his humanity, Jesus is our brother and through his divinity he links us in chains of unbroken love to the throne of God. He is the God-man.

John 3:16 also tells us this, in that the word "given" is used. He was not "lent". He is a permanent gift to humanity. He was, is, and always will be, the Son of Man and the Son of God.

In the bible, the word of God, we find the prophecies concerning the Messiah, Jesus. If we focus on a belief in God to the exclusion of Jesus we in fact show that we do not believe in God. Jesus (again John 14) said "You believe in God; believe also in me". If we believe in God we will believe in the one he gave to us. We will believe his prophecies. We will believe the words of Daniels vision where the date of Jesus ministry of 3 and a half years was given.

A belief in Jesus is a necessary and vital point in believing in God. If you reject Gods Son you reject God. "Whoever listens to you, listens to me; whoever rejects you rejects me; but whoever rejects me rejects him who sent me." (Luke 10:16).

I believe this should adequately address the reasons why our belief is in Jesus. By believing in Him, we believe in the One who sent Him.

Believing in Jesus means believing that through him we can get the remission of our sins. This is one aspect that is emphasized by saying "believe in Jesus" rather than just "believe in God", which is more unspecific.

It is one thing to believe that Jesus existed as a historical person (which many scholars do). It is another thing to believe in His Divinity (which most non-Christian scholars usually don't).

When Christians talk about "believing in Jesus", this is more likely than not what they mean.

All three words of "believe in Jesus" are important.

1. Believe. Belief as it is used here is more than intellectual assent to a proposition or recognition of a historical fact. The scope of the meaning of belief that is here intended includes trust and obedience. James says that "faith without works is dead". This is an active belief.

17 In the same way, faith by itself, if it is not accompanied by action, is dead.

18 But someone will say, “You have faith; I have deeds.”

Show me your faith without deeds, and I will show you my faith by my deeds. 19 You believe that there is one God. Good! Even the demons believe that—and shudder.

20 You foolish person, do you want evidence that faith without deeds is useless? 21 Was not our father Abraham considered righteous for what he did when he offered his son Isaac on the altar? 22 You see that his faith and his actions were working together, and his faith was made complete by what he did. 23 And the scripture was fulfilled that says, “Abraham believed God, and it was credited to him as righteousness,” and he was called God’s friend. 24 You see that a person is considered righteous by what they do and not by faith alone. (James 2:17-24)

2. Jesus. Among the most central beliefs in orthodox Christianity is that Jesus is God come to us in the flesh, the incarnation. Some religions propose a belief in God but deny that God has a son or deny that God can take on human form. The Apostle John made this the essential test for discerning whether a spirit or message is from God. You must acknowledge the incarnation.

1 Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. 2 This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, 3 but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world. (1 John 4:1-3)

(NOTE: All churches that accept the Nicene creed (or at least the Apostle's creed) believe in a physical incarnation. Many heretical groups denied this or parts of this, such as those that believed in Docetism, Ebionism, Arianism, Apollinarianism, Nestorianism, Eutychianism, Monophysitism, and Monothelitism. Some modern denominations also do not accept this, so would not be considered orthodox. See https://mbcpathway.com/2018/08/31/incarnational-heresies/)

3. in. This is the hardest of the three words. As a new believer, I had a hard time unerstanding this preposition as it applied to statements in the Bible. In has to do with abiding in Christ.

1 “I am the true vine, and my Father is the gardener. 2 He cuts off every branch in me that bears no fruit, while every branch that does bear fruit he prunes so that it will be even more fruitful. 3 You are already clean because of the word I have spoken to you. 4 Remain in me, as I also remain in you. No branch can bear fruit by itself; it must remain in the vine. Neither can you bear fruit unless you remain in me.

5 “I am the vine; you are the branches. If you remain in me and I in you, you will bear much fruit; apart from me you can do nothing. 6 If you do not remain in me, you are like a branch that is thrown away and withers; such branches are picked up, thrown into the fire and burned. 7 If you remain in me and my words remain in you, ask whatever you wish, and it will be done for you. 8 This is to my Father’s glory, that you bear much fruit, showing yourselves to be my disciples. (John 15:1-8)

You must also take into this the meaning of Jesus' name. From Matthew 1:21 we have:

"She will give birth to a son, and you are to give him the name Jesus, because he will save his people from their sins.”

A person must believe that it is through this belief, this faith in Jesus, that they are saved, not through any other cause, especially human effort and religious ritual. One passage that drives this home:

“For through the law I died to the law so that I might live for God. 20 I have been crucified with Christ and I no longer live, but Christ lives in me. The life I now live in the body, I live by faith in the Son of God, who loved me and gave himself for me. 21 I do not set aside the grace of God, for if righteousness could be gained through the law, Christ died for nothing!” (Galatians 2:19-21)

Righteousness through the law, as Paul above speaks, means following religious, moral rules so well on your own that you earn God's acceptance. It can't be done, though many religions say it can.

(There is a debate between Protestants and Catholics as to whether personal acts of righteousness in obedience to Christ are part of this saving faith or follow as a consequence from it. That is another question, covered a lot on this site.)

Origin of the Phrase

I am unable to determine the origin of the phrase. However, it may be related to the Apostle's creed, where it begins the second section:

I believe in God, the Father almighty,
      creator of heaven and earth.

I believe in Jesus Christ, his only Son, our Lord,
      who was conceived by the Holy Spirit
      and born of the virgin Mary.
      He suffered under Pontius Pilate,
      was crucified, died, and was buried;
      he descended to hell.
      The third day he rose again from the dead.
      He ascended to heaven
      and is seated at the right hand of God the Father almighty.
      From there he will come to judge the living and the dead.

I believe in the Holy Spirit,
      the holy catholic* church,
      the communion of saints,
      the forgiveness of sins,
      the resurrection of the body,
      and the life everlasting. Amen.