Immediate context:
NKJ Galatians 4:31-5:1 So then, brethren, we are not children of the bondwoman but of the free. 5:1 Stand fast therefore in the liberty by which Christ has made us free, and do not be entangled again with a yoke of bondage.
Back to the beginning and proceeding forth:
NKJ Galatians 1:1-2 Paul, an apostle (not from men nor through man, but through Jesus Christ and God the Father who raised Him from the dead), 2 and all the brethren who are with me, To the churches of Galatia:
NKJ Galatians 1:6 I marvel that you are turning away (BDAG Lexicon: transferring, changing, deserting) so soon from Him who called you in the grace of Christ, to a different (another) gospel,
NKJ Galatians 3:1 O foolish Galatians! Who has bewitched you that you should not obey the truth, before whose eyes Jesus Christ was clearly portrayed among you as crucified? 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? 4 Have you suffered so many things in vain-- if indeed it was in vain?
NKJ Galatians 3:15 Brethren, I speak in the manner of men: Though it is only a man's covenant, yet if it is confirmed, no one annuls or adds to it.
NKJ Galatians 3:26-29 For you are all sons of God through faith in Christ Jesus. 27 For as many of you as were baptized into Christ have put on Christ. 28 There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. 29 And if you are Christ's, then you are Abraham's seed, and heirs according to the promise.
NKJ Galatians 4:6-7 And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, "Abba, Father!"
7 Therefore you are no longer a slave but a son, and if a son, then an heir of God through Christ.
NKJ Galatians 4:8-11 But then, indeed, when you did not know God (= in the past), you served those which by nature are not gods. 9 But now after you have known God, or rather are known by God, how is it that you turn again to the weak and beggarly elements, to which you desire again to be in bondage? 10 You observe days and months and seasons and years. 11 I am afraid for you, lest I have labored for you in vain.
NKJ Galatians 4:12 Brethren, I urge you to become like me, for I became like you. You have not injured me at all.
NKJ Galatians 4:14-21 And my trial which was in my flesh you did not despise or reject, but you received me as an angel of God, even as Christ Jesus. 15 What then was the blessing you enjoyed? For I bear you witness that, if possible, you would have plucked out your own eyes and given them to me. 16 Have I therefore become your enemy because I tell you the truth? 17 They zealously court you, but for no good; yes, they want to exclude you, that you may be zealous for them. 18 But it is good to be zealous in a good thing always, and not only when I am present with you. 19 My little children, for whom I labor in birth again until Christ is formed in you, 20 I would like to be present with you now and to change my tone; for I have doubts about you. 21 Tell me, you who desire to be under the law, do you not hear the law?
NKJ Galatians 4:31-5:1 So then, brethren, we are not children of the bondwoman but of the free. 5:1 Stand fast therefore in the liberty by which Christ has made us free, and do not be entangled again with a yoke of bondage. 2 Indeed I, Paul, say to you that if you become circumcised, Christ will profit you nothing. 3 And I testify again to every man who becomes circumcised that he is a debtor to keep the whole law. 4 You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace. 5 For we through the Spirit eagerly wait for the hope of righteousness by faith. 6 For in Christ Jesus neither circumcision nor uncircumcision avails anything, but faith working through love. 7 You ran well. Who hindered you from obeying the truth?
Stopping here for now.
You mentioned what is clear vs. what is unclear. What's unclear about who Paul is writing to here and what his concern was for them?
I think it's important to let all Scripture say what it says and to figure out how to make sense of it. I'm very familiar with teachings of eternal security, and the construct once-saved-always-saved and doctrines of justification and sanctification and rewards and such that are used to impose other Scriptures over Scriptures like these. I had them drilled into me for substantial parts of thousands of hours under intense teaching. Now I just study Scripture.
BTW, in response to your question about "in Christ" - once again I just let Scripture explain it, so I've highlighted a couple references to the phrase above in blue. Since it's mostly a phrase Paul uses, I think a partial if not good explanation of it is hereinabove - especially in 3:26-29, but also in the context of what Paul is speaking about in Galatians to his brethren and his little children.