From the Conclusion

Bonhoeffer correctly saw the enemy of Jesus’ gospel. It is anyone who preaches salvation without repentance from sin: “Cheap grace is the preaching of forgiveness without requiring repentance.” (Cost of Discipleship, supra, at 44.)

You can take that cheap grace route. It is easy. There is no agonizing to enter. It has no sacrifice you must suffer. Your salvation-transaction can all be over and done in ten seconds, as Thieme says. It is painless and free. But if you are wrong, and you rely to your detriment upon it as enough when it is not enough, you will find out only when it is too late that you were deadly wrong.
Why not follow instead Jesus’ literal words? He said you can go to heaven-maimed or hell-whole. (Mark 9:42-47.) You can repent from sin, turning in abhorrence from it, or you will perish. (Luke 13:2-5.) He said if you do not have good fruit, you are going to be cut down and thrown in the fire. (Matt. 7:19.) A branch “in me” without fruit is “taken away,” thrown “outside” to be “burned.” (John 15:1-6.)

What kind of fruit? One example Jesus clearly taught is that if you do not have charity for others, you go to eternal fire, but if you are charitable, you are resurrected to eternal life. (Parable of the Sheep and the Goats. Matthew 25:30-46.)

It was just as Jesus elsewhere said: those who do “good things” resurrect to eternal life, but those who do evil things go to eternal damnation. (John 5:28-29.)

What kind of evil things lead to damnation?

Jesus said if you do an evil thing like “fail to forgive” as did the Unmerciful Servant, Jesus said God will treat you precisely like the master treated that servant in the parable: once God has already forgiven you, if you then do not forgive others, God will revoke your forgiveness, send you to Hell and punish you in torment, making you pay forever the previously forgiven debt of sins. (Matthew 18:23-35.)

If you do an evil thing like starting to “beat” your fellow Christian and “eat and drink with the drunken,” your prior service as a good servant is forgotten. Instead you will be sent “outside in darkness” to “suffer weeping and gnashing of teeth” with “unbelievers.” (Luke 12:41-48; Matthew 24: 44-50.)

What must one do then to obtain eternal life? When asked that same question, Jesus said “to enter life obey the commandments,” and He rattled off nine of the Ten Commandments. (Matthew 19:16-26; Mark 10:17-31; Luke 18:18-26.)
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But you ask: ‘Won’t believing save me alone instead?’

No, it won’t. If you “keep on obeying unto” Jesus, then you “should not ever ever die” (John 8:51) and “should not perish, but should have everlasting life.” (John 3:16.)

In fact, Jesus was adamant that professing belief in Him as Lord is meaningless unless you also obey Him. Jesus said those who call Him ‘Lord Lord’ but “do not do what I say” have a flawed concept of what it means to say that He is Lord. (Luke 6:46.) If you call Him Lord “but do not do His will” Jesus will tell you “I never knew you.” (Matt. 7:21.) Jesus said those who profess to want to obey Him, but do not actually do so are lost. But those sinners who repent and obey will enter heaven instead of those who merely profess they will obey Him but do not do so. (Parable of the Two Sons. Matthew 21:28-31.)

Jesus means obedience is not optional for those who call Him Lord. Salvation is gained or lost depending on actual obedience, not mere profession one way or the other.
In fact, Jesus taught one who has wrong belief, such as a member of the Samaritan sect, has “eternal life” when they obey God’s commandments such as to save a life along the road. But those who believe in the true God and profess to teach God’s commands, yet do not do them, like the Levite and religious ruler in the Parable of the Good Samaritan, are lost. (Luke 10:25 ff.)

This parable has a blunt point: salvation depends on whether you obeyed God’s commands. It does not turn alone on what you believed and taught, no matter how right or wrong was your doctrine.

You will ask: ‘Can I at least depend on Christ’s atonement covering me if I believe I am a sinner and the blood of Christ washes me?’

Jesus says emphatically no! Atonement is not magic.

Jesus taught instead you can appropriate atonement if you first obtain reconciliation with the ones you have offended. (Matt 5:22-26.)
You are only right with God — justified — when you have repented from sin, like the Publican in the Parable of the Pharisee and the Publican. It was the publican who went home “justified.” (Luke 18:9-14.)

By repentance from sin and turning back to your father is how you go from being “dead and lost” to being “alive again.” (Parable of the Prodigal Son. Luke 15:17-24, viz. verse 24.) Repentance from sin is thus the condition for the synonymous expression that “to enter heaven” one must be “born again.” (John 3:3.)

You final reward of obedience is that all who obeyed Jesus’ commandments “shall have the right” to eternal life.
"Happy [are] the ones doing His commandments, so that their right will be to the tree of life, and they shall enter by the gates into the city." (Rev 22:14)