Matthew 24:9-13
9 Then they will deliver you up to tribulation and kill you, and you will be hated by all nations for My name's sake.
10 And then many will be offended, will betray one another, and will hate one another.
11 Then many false prophets will rise up and deceive many.
12 And because lawlessness will abound, the love of many will grow cold.
13 But he who endures to the end shall be saved.
I don't think the passage indicates that salvation is dependent upon the endurance of a genuine believer who is under persecution but that the pressures of persecution will reveal the reality of a person's claim to be saved ("many will be offended," "the love of many will grow cold." See also 1 John 2:19). The one who endures to the end is the one who is (and so shall be) saved.
I don't see anywhere in this passage where Paul indicates that a genuine believer may lose their salvation. He simply notes the result of saving faith: A faith that perseveres, established and firm in the hope of the Gospel, is a faith that produces a holy life, free from blemish and accusation. A faith that does not persevere is a false faith, the faith of a "tare," that does not hold firm unto the end. If the Colossians claiming to be saved possessed such a faith, they could not expect to remained unmoved from the hope of the Gospel. This is how I understand Paul's words here. And my understanding appears to be reinforced by Paul pointing out that the Colossians had already been reconciled to God; they were not, by dint of their perseverance, securing their reconciliation to God. This certainly seems to coincide well with Paul wrote to the Galatian believers:
Galatians 3:2-3
2 This only I want to learn from you: Did you receive the Spirit by the works of the law, or by the hearing of faith?
3 Are you so foolish? Having begun in the Spirit, are you now being made perfect by the flesh?
Coming out of the OT, this passage has no bearing that I can see upon the circumstance of a person under the New Covenant established in and through Christ. It is precisely because the "righteous" person in the OT could not achieve perfect righteousness that animal sacrifices had to be made and Christ, finally, had to die at Calvary. (Romans 3:19-23; Romans 5:12)
Was Paul describing believers when he wrote what he did from Romans 1:18 to the end of the chapter? Yes, he starts off his letter acknowledging to whom it he is writing, but this doesn't mean that everything he describes in his letter is in direct reference to the Roman believers. This is certainly true in the last part of chapter 1 and it is true also in the passage from chapter 2 you cite above. Neither passage describes the Romans to whom Paul was writing. These descriptions seem, rather, to be set in distinct contrast to them and as examples of what to avoid. In chapter 2, Paul is speaking (throughout the whole chapter, really) particularly to hypocritical teachers:
Romans 2:1
1 Therefore you are inexcusable, O man, whoever you are who judge, for in whatever you judge another you condemn yourself; for you who judge practice the same things.
It's worth noting, too, that Paul doesn't use terms here that stipulate he is referring to Roman believers. Instead, he resorts to "O man" and "whoever you are," not "brethren" as he so often does in other places. At the end of the chapter he continues to describe who he means and his language does not give me cause to think he was referring to Roman believers:
Romans 2:17
17 Indeed you are called a Jew, and rest on the law, and make your boast in God,
Romans 2:24
24 For "the name of God is blasphemed among the Gentiles because of you," as it is written.
I don't see how it makes sense to think Paul is referring to Roman believers here. It seems to me that his descriptions above make it quite evident that he is not.
Paul isn't indicating that good works obtain one's salvation in Galatians 6:9. That would directly contradict his explicit statements in Ephesians 2:8-9 and Titus 3:5. It seems to me, then, that by "a harvest" Paul means the fruit of the good deeds sown by believers: Sowing mercy produces a harvest of mercy (Matthew 5:7); sowing friendliness produces a harvest of friends (Proverbs 18:24); sowing diligence produces a harvest of recognition from superiors (Proverbs 22:29), and so on.
The quotation above from Galatians 3 rather makes my case for me. It clearly excludes works as a means of salvation. As Paul explained in Titus 3:5, the Spirit gives spiritual life to a person entirely independently of their works. Paul simply repeats this fact to the Galatians.