faith

Definition:

In general, the term "faith" refers to a belief, trust or confidence in someone or something.

To "have faith" in someone is to believe that what he says and does is true and trustworthy.
To "have faith in Jesus" means to believe all of God's teachings about Jesus. It especially means that people trust in Jesus and his sacrifice to cleanse them from their sin and to rescue them from the punishment they deserve because of their sin.
True faith or belief in Jesus will cause a person to produce good spiritual fruits or behaviors because the Holy Spirit is living in him.
Sometimes "faith" refers generally to all the teachings about Jesus, as in the expression "the truths of the faith."
In contexts such as "keep the faith" or "abandon the faith," the term "faith" refers to the state or condition of believing all the teachings about Jesus.

In some contexts, "faith" can be translated as "belief" or "conviction" or "confidence" or "trust."
For some languages these terms will be translated using forms of the verb "believe." (See: rc://en/ta/man/jit/figs-abstractnouns)
The expression "keep the faith" could be translated by "keep believing in Jesus" or "continue to believe in Jesus."
The sentence "they must keep hold of the deep truths of the faith" could be translated by "they must keep believing all the true things about Jesus that they have been taught."
The expression "my true son in the faith" could be translated by something like "who is like a son to me because I taught him to believe in Jesus" or "my true spiritual son, who believes in Jesus."

05:06 When Isaac was a young man, God tested Abraham's faith by saying, "Take Isaac, your only son, and kill him as a sacrifice to me."
31:07 Then he (Jesus) said to Peter, "You man of little faith, why did you doubt?"
32:16 Jesus said to her, "Your faith has healed you. Go in peace."
38:09 Then Jesus said to Peter, "Satan wants to have all of you, but I have prayed for you, Peter, that your faith will not fail.