Douglas J. Moo, Hoti, and Galatians 4:6

Greek: ὅτι δέ ἐστε υἱοί, ἐξαπέστειλεν ὁ θεὸς τὸ πνεῦμα τοῦ υἱοῦ αὐτοῦ εἰς τὰς καρδίας ἡμῶν, κρᾶζον· Αββα ὁ πατήρ. (SBLGNT)

There is some debate about whether ὅτι in Galatians 4:6 is demonstrative or causal.

NWT: Now because you are sons, God has sent the spirit of his Son into our hearts, and it cries out: “Abba, Father!”

HCSB: And because you are sons, God has sent the Spirit of His Son into our hearts, crying, “Abba, Father!”

CEV: Now that we are his children, God has sent the Spirit of his Son into our hearts. And his Spirit tells us that God is our Father.

ESV: And because you are sons, God has sent the Spirit of his Son into our hearts, crying, “Abba! Father!”

Tyndale Bible: Be cause ye are sonnes God hath sent the sprete of his sonne in to oure hertes which cryeth Abba father.

While it seems that ὅτι is definitely causal in this case, Douglas J. Moo writes the following (Galatians, Baker Exegetical Series):

The initial ὅτι (hoti, because) is most naturally taken in a causal sense, and this also fits the sequence of Paul’s argument: we have received the adoption as sons, and because we are now sons, we also have the Spirit. The only problem with this interpretation is that the claim that possession of the Spirit proceeds from the status of sonship appears to contradict Paul’s usual sequence, which is just the opposite. Especially important is Rom. 8:14–17, a passage with many obvious similarities to Gal. 4:4–6

Moo quotes the verses in Romans, then he professes:

Further, earlier in Galatians Paul has assumed that the Spirit marks the beginning of Christian experience (3:3). For these reasons, some interpreters suggest that this initial ὅτι might have a “declarative” function and depend on an implied assertion such as “to prove that”; see NAB: “As proof that you are children, God sent the spirit of his Son into our hearts, crying out, ‘Abba, Father!’” (see also Lietzmann 1932: 25; Dunn 1993a: 219; Moule 1959: 147; Rohde 1989: 173). However, this is not the obvious way to understand ὅτι, and it is doubtful if we need to introduce such a reading in order to preserve Pauline consistency. A careful reading of Rom. 8:14–17 shows that Paul is not clearly arguing for a sequence of Spirit—sonship. Having the Spirit means that one is a “son”; but Paul is not clearly saying that the Spirit confers sonship. Nor does Paul’s claim that the Spirit marks the “beginning” of Christian experience (3:3) mean that the Spirit must be absolutely prior to all other Christian blessings. Paul wants to associate the status of sonship with the gift of the Spirit, but claiming that Paul teaches a strict temporal or logical sequence between them would be overreading this text and others (see, e.g., R. Longenecker 1990: 173; Martyn 1997: 391; Schreiner 2010: 272).[18] The “sequence” of sonship and Spirit in various texts in Paul is thus probably dictated more by rhetorical than theological concerns.

In addition to the commentators that Moo invokes, see John Eadie's extended comments on Galatians 4:6.