Verses 38-41 form a unit. 42 is transitional. Verses 43-50 form a second unit.
There is no conjunction or particle. John interrupts Jesus. What he says indicates that he was not sure whether he had done the right thing. "We were trying to prevent him." Note that he says "us" not "you." He judges simply by membership in the circle of the disciples, who had been commanded to drive out devils.
"Stop preventing him" likely indicates that the man is still doing so. "In the next moment" is "in the same breath." "Say anything bad" or "speak evil" means "he believes in Me."
This verse states a Christian axiom. Note Matthew 12:30, an opposite thought, found in a context of those who reject Jesus. The use of "us" in verse 40 includes Jesus. In verse 38 it obviously excludes Jesus. Both this verse and Matthew 12:30 tell us that no one can be neutral to Jesus.
This explanation cites a specific case which however is general. Intended sense: a deed, no matter how insignificant, done "on the ground that you are Christ's" will always be rewarded. That is the attitude they should have toward such as are mentioned in verses 38-39, not an attitude of exclusion.
The very opposite of verse 37 and a glaring contrast with verse 41. It is a general statement covering all instances of this kind. "Cause to sin" is "to entrap." NEB: "Leads astray." "Believe in Me" surely means faith in Christ. Little children can believe. KJV, RSV and NASB make the apodosis a contrary to fact condition "and had been cast into the sea." Grammars argue about the meaning of "if" or "that" here. It makes little difference. The meaning is clear: "Better to die a violent and cruel death than to cause one of these little believers to sin." Christians must control their flesh so as not to mislead others.
Verses 43-47 (with 44 and 46 omitted) comprise a three-fold warning to the Christian with reference to self:
"Their" is the damned in hell. Hell is pictured as everlasting internal corruption and putrefaction and external torture. Look at 2 Thessalonians 1:9, eternal objects of God's wrath.
Verses 42-48 are a stern warning. Who is not guilty and who does not need warning?
This verse is a Gospel promise. It explains, and denotes the way out for the condemned sinner who is conscious of his guilt. "Everyone" denotes every Christian. "Will be salted" means "will be cleansed, will be purified." "With fire" is dative of means and denotes the Word of God. Look at John 15:2-3. Note that "fire" has a radically different meaning here than in verse 48. For a parallel example compare Luke 3:16-17, where "fire" has distinctly different meanings. By the way, the difference between Lutheran and Reformed theology is evident in the explanation of verse 49. Lenski interprets "fire" as the Word and the Spirit. But Hendriksen: "A fiery trial will come upon everybody, for the purpose of purification." Obviously he says this because he does not believe that the Word is the means of grace.
The first three words are axiomatic. Everyone will agree with this statement. Note that predicate adjective is placed for emphasis: "The salt is truly good." And likely the article indicates a particular type of salt, the Word and the Spirit.
Next follows a rhetorical question, a good teaching device. Without salt there can be no seasoning. Without the Word and Spirit there can be no forgiveness, no battle again sin, no everlasting life. By the way, this sentence does away with the idea: "Once in grace, always in grace."
The final sentence is a compound sentence, with compound imperatives, contrasting reflexive and reciprocal action. A connecting word "and so" in the Greek, not translated here, likely means "and thus." Only when we have the Word and Spirit in ourselves can we have peace among ourselves. We have come full circle from verses 34 and 35. The only cure for pride, prejudice, offense and living in sin is the salt of the Word and the Spirit. This will keep us at peace with each other. Note again that verses 48-50 are found only here in Mark.
Stoeckhardt: He who tames his members with the help of the Holy Spirit, holds himself within bounds, does not give sin free reign, he keeps his faith and a good conscience, keeps his body and should to everlasting life. The Lord demands a painful offering from His disciples, the offering of their own members . . . Christians, who are sanctified and make progress in sanctification through the Word and Spirit of the Lord, must always have this salt and with God's Word and Spirit must part ways with the rotten deeds of the world, rather than to permit themselves to be lured into sin by the world, but must practice mutual peace and must not consider themselves superior to others.