Sin

God is holy and perfect. Hence His standard for living is holiness and perfection. It is impossible for us to be perfect and everyone knows it. Anyone who is honest with themselves admits to not being perfect. This imperfection we all experience on a daily basis is what the Bible is talking about when it refers to "sin".

The word "sin" actually comes from a Greek word that was used as a measurement in archery. Essentially people would measure the distance from where the arrow hit the target to the bulls-eye in sins. Imperfection...get it? However, author Randy Newman in his book Questioning Evangelism states that the archery analogy is an incomplete one (and I agree with him). He gave me permission to quote him:


“Sin is simply missing the mark,” they say. “The same Greek word for sin is used as an archery term, so we’re all just ‘target-missers.’”
Well, the same Greek word might be used, but the two concepts couldn’t be further apart. When the Bible describes the nature of our rebellion against God, it paints an uglier picture than our simply missing a bull’s eye (see Rom. 3:10–18). Rather than aiming carefully at God’s target, we turn our backs and shoot arrows everywhere else. Wanting to please ourselves, we ignore the true bull’s eye and set our affections on seductive targets that cannot satisfy, sanctify, or save. We are not primarily target-missers; we are self-centered false-target worshippers.
I wouldn’t suggest saying any of that to a non-Christian, but I would avoid the archery illustration. Following such faulty reasoning, a thoughtful seeker might wonder why God would go to all of the trouble of the Cross simply because we aren’t spiritual Robin Hoods.

That's pretty clear. Thanks Randy! One could be passively indifferent toward God's standard (unaware of or apathetic towards it) or actively rebellious against it (you know and understand it, yet choose to reject it) and be classified as a sinner.

I know this seems bleak, but hang in there. I think the next phrase I'll define is "the Gospel." That's where the good news comes in!

Christian

A Christian is anyone who realizes that they are not good, and subsequently fully trusts in Jesus Christ for the forgiveness of all they've done wrong and for their assurance of eternity in Heaven with God. Once someone realizes how much God loves them in providing a way for them to be forgiven from all the wrong they've done, and trusts Christ to forgive them from their sins and rescue them from eternal separation from Him and torment in Hell, he or she then strives to become more like Christ as their life goes on. Doing so should be a thankful response to the grace that they have been shown in being forgiven by God and through God, and not as a means for obtaining salvation by their own merit. Even the very ability or power to do anything pleasing to God comes from God (namely the Holy Spirit. Don't worry, we'll get to defining the Trinity)! It is impossible for mankind to be "good enough". We need the perfection of Christ to be applied to our lives in order to have a personal relationship with, or be in the presence of, a Holy God. This is what a person who claims to be a Christian is affirming whether they realize it or not.

The word Christian literally means "follower of Christ" or "little Christ".

I feel like there's way more the say here, but I'll leave it at this for now.

Two additions based off your responses:

1. To be a Christian one must acknowledge the deity of Jesus Christ (know that He was, is, and always will be God).

2. To be a Christian one must also believe that Jesus Christ was resurrected bodily (He came back to life after being dead for 3 days). He was confirmed dead by a professional executioner, was buried for 3 days, and after He came back to life more than 512 people saw Him in person before He ascended to heaven. That is a lot of people to back up a claim!

Theology

The study of God.

Theologian - one who studies (and often writes or teaches about) God for a living.

You will often hear people talking about bad theology vs. good theology. Basically bad theology is arrived at when someone approaches their study of God with an agenda or a preconceived notion of who or what they want God to be. They pick and choose Scriptures or quotations (often out of context) that support what they believe. Good theology is arrived at when people examine and study the whole Bible and wrestle with who God is, regardless of their own perceptions and biases, trying to understand Him.

To our shame, Christians often get hostile with each other over theological issues. At the same time it is biblical to challenge our fellow Christians in love when they are in error. i.e. Acts 17:11, Galatians 2:11-21. The hardest part is doing it in love.

Amen

When said at the end of praying, it means so be it.
When other people say it to you, it means, "I agree with that!"

What else you may hear
"Amen and AMEN" = "I REALLY agree with that!"
"Amen brother (or sister)!" = "I agree with you fellow Christian! I'm glad we can agree on this issue!"