1. THE TITLE-CHANGER: doesn’t understand the gospel, and therefore never believed. 

Jesus explains it here (Matthew 13:19, Luke 8:12)
This describes thousands who sit in church weekly, were born into a Christian family, or otherwise claimed the name “Christian” but have never believed in Jesus for eternal life. We can only call them “Christian” in the looses sense of the word. (Luke 13:23–24) 
In this case, becoming an atheist is not a change of theological position but only a change of title. The title “Christian” never fit this person in the first place.

2. THE MID-SEASON QUITTER: believes in Jesus and pursues discipleship for a while. When faced with temptation or suffering he gives up.

Jesus explains it here (Matthew 13:20–22, Luke 8:13)
They “receive the word with joy,” and “believe for a while.” This person has believed Jesus and therefore has eternal life. (John 3:36, 5:24) However, they only pursue a Godly lifestyle for a time. 
The bible tells us that some believers do not endure in faith or good works. (2 Timothy 2:11–12) It’s possible for a believer to “deny Jesus.” It says that Jesus will, ”deny him” in return. That doesn’t mean he will deny them eternal life. It means he will deny him reward and friendship.
The very next verse tells us what Jesus’ response will be. (2 Timothy 2:14) Even when he denies Jesus and is “faithless,” Jesus “remains faithful.” Therefore Jesus will reprimand and discipline the Mid-Season-Quitter, but he WILL NOT take away his eternal life. The Mid-Season-Quitter will still be saved (1 Corinthians 3:15, Romans 4:5)

3. THE CHRISTIAN SIN-CHASER: believed in Jesus, but doesn’t pursue discipleship, and gets distracted by sin.

Jesus explains it here (Luke 8:14, Matthew 13:22)
The bible tells us that there are believers who never grow beyond spiritual infancy before they die. (1 Corinthians 3:3, 11:30, Hebrews 5:12) In the same way that the Mid-Season-Quitter can “deny Jesus” the Sin-Chaser can as well. Notice in this verse where the denial by Jesus is happening. (Matthew 10:33) It happens in Heaven. This person who once believed, but then denied Jesus has to be in Heaven to be denied by Jesus. So his soul will be saved even though his body will suffer the consequences of sin. (1 Corinthians 5:5)
There comes a time when God turns the Christian Sin-Chaser over to evil desires and thoughts. (Romans 1:24, Psalm 81:12) This person causes the Holy Spirit so much grief (Ephesians 4:30) that God will allow them to do and think whatever they want. In this case, the mind is darkened and judgment is clouded. (Ephesians 4:17) This has consequences. The Christian Sin-Chaser will lose rewards in Heaven (Matthew 6:19–21,1 Corinthians 3:10–15) but he will be eternally saved. 
All these are pretty sad. Good News. There is another option!

BONUS / THE ENDURER: believes, pursues discipleship until he dies, and is rewarded in Heaven.

Jesus explains it here (Matthew 13:23, Luke 8:15)
Believers who also obey Christ until their lives are up will gain reward in Heaven. (Matthew 5:12, Luke 6:23, Revelation 22:12, Philippians 3:14, Hebrews 11:26, Matthew 16:27, Daniel 12:3, 1 Corinthians 3:8, 2 Corinthians 9:6, Luke 6:22–23, 1 Peter 5:4, 1 Thessalonians 2:19, 2 Timothy 4:8, Ephesians 2:6, John 14:3, John 17:24, 1 John 3:2, Revelation 22:4, Philippians 3:20–21, 1 Corinthians 15:42–44, 2 Corinthians 5:1–4)