[rough draft – in progress]

Believing in Jesus promotes good

  1. Christianity promotes love and selflessness.

Golden rule; turn the other cheek; love those who hate you; inasmuch as ye have done it unto one of the least of these my brethren.

It's difficult to overstate the power of these ideals.  These ideas exist (often to a somewhat lesser degree) in other cultures or religions, but they are persistently and emphatically stressed by Jesus Christ.  Much of the good that has been done in the world was motivated, at least in part, by these ideals.
  1. Idea of a resurrection or eternal life diminishes existential anxiety.

  2. Attending Church is healthy, perhaps above and beyond normal social association. (source)

Believing in Jesus may also promote some harmful attitudes

A belief in the teachings of Jesus may:

  1. Hinder introspection and the quest for Truth.

    The desire to maintain a belief in God and Christ tends to hinder a person from searching for and fairly evaluate the evidence that might contradict that belief.

    Mark 4:28-29

    Truly I say to you, all sins will be forgiven the sons of men, and whatever blasphemies they utter; but whoever blasphemies against the Holy Spirit never has forgiveness, but is guilty of an eternal sin.

    Mark 8:11-12

    The Pharisees came and began to argue with him, seeking from him a sign from heaven, to test him. And he sighed deeply in his spirit and said, “Why does this generation need a sign? Truly, I say to you, no sign shall be given to this generation.”

    Mark 16:16

    He who believes and is baptized will be saved; but he who does not believe will be condemned.

    This avoidance of introspection seems to be directly correlated with level of belief (i.e., JW > Mormon > SDA > Evangelical > Catholic > Protestant > UU). Generally, the stronger the belief in God and Jesus, the less likely a person or group seems to be willing to confront facts which might potentially undermine that belief.

  2. Promote second-coming nihilism and anxiety.

Whereas the belief in the resurrection may quell anxiety

Anxiety about burning in hell? Judgement??

Belief in the imminent return of Jesus with accompanying cataclysms (see Matthew 24 and the Book of Revelation) is woven deeply into Christianity

  1. An attitude of “doctrine over person”

    • John taught that if a person believed in a different doctrinal version of Jesus Christ (i.e., the gnostic doctrine) that they should not say “hi” to them or invite them into your house. 2 John 1: 7-11

    • Jesus Christ discouraged a disciple from burying his father with the statement “let the dead bury their dead”. This teaching is used by believers to prioritize Church and religion over family and non-believers.

    • Luke 9:61-62: Another said, “I will follow you, Lord; but let me first say farewell to those at my home.” Jesus said to him, “No one who puts his hand to the plow and looks back is fit for the kingdom of God.”

    • Luke 14:26

      If any man come to me, and hate not his father, and mother, and wife, and children, and brethren, and sisters, yea, and his own life also, he cannot be my disciple.

  2. Harmful sexual attitudes

Religious belief is directly and strongly correlated with teenage pregnancy.

Plot

Probably the single most important predictor of the health and success of the mother and child is predicted by religious belief. Christianity deals with sex via guilt and shame.

  1. Division in families over belief

If everyone does not believe in Jesus (in the same way), this may generate conflict.

Matthew 10:35-37:

> For I have come to set a man against his father, and a daughter against her mother, and a daughter-in-law against her mother-in-law; and a man's foes will be those of his own household. He who loves father or mother more than me is not worthy of me; and he who loves son or daughter more than me is not worthy of me.

Luke 18:29-30: And he said to them, "Truly, I say to you, there is no man who has left house or wife or brothers or parents or children, for the sake of the kingdom of God, who will not receive manifold more in this time, and in the age to come eternal life."
  1. Disunity

Belief in the unverifiable promotes disunity.

Schisms.

I can be skeptical of God, and be inspired by Jawaharlal Nehru to do good, love others, and be kind to children without the baggage that belief in God and Jesus bring.

  1. The idea of the inherent superiority of certain groups of people

Story of the Canaanite woman: Matthew 15:22-28

But he answered and said, I am not sent but unto the lost sheep of the house of Israel

  1. An acceptance of slavery

(mostly a past harm)

  1. The warping of reality

Ultimately, the doctrine that Jesus was infallible and morally right in every instance must be preserved at any cost—the cost of mis-interpreting or twisting the clear and straightforward meaning of his words.

Generally, the manner in which this is most easily accomplished is to villify the messenger—the messenger is ungenerous, ignorant, or guilty of twisting the “real meaning” with malicious and Satanic intent.

See also

A comment on /r/exmormon discussing similar points, some in slightly different ways.