In the book of Galatians, Paul quotes Genesis
15:6 and Habakkuk 2:4 to establish the fundamental soteriological doctrine of
justification before God by faith alone. 
Genesis 15:6 is quoted in Galatians 3:6, while Habakkuk 2:4 is quoted in
Galatians 3:11.  Galatians 3:1-4:11
provides arguments in favor of the propositions Paul stated in Galatians
2:15-21.  Galatians 3:1-18 proves
that righteousness is received apart from the law.  Within 3:1-18, 3:6-14 provides arguments from the Old
Testament establishing the truth of justification by faith apart from the law.[i]  Paul points out, first of all, that the
truth that one is justified in the sight of God apart from the law (2:16) is
established because Abraham was accounted righteous, receiving the imputed
righteousness of the Messiah, through the sole instrumentality of faith (3:6).[ii]  Consequently, believers, “they which
are of faith,” rather than law-keepers, “are the children of Abraham”
spiritually (3:7).  Those who
believe as Abraham did become the recipients of the redemptive blessings
associated with the patriarch. 
Indeed, the Old Testament had forseen that God would justify Gentiles,
non-lawkeepers, through faith, for God had promised Abraham all nations, not lawkeeping
Jews only, blessing through his Seed, the Messiah.[iii]  Consequently, all those who are of
faith receive the Abrahamic blessing (3:9).  Indeed, none of the sons of Adam can receive salvation
through obedience to the law, for the legal standard is continual, perfect,
sinless obedience, but all have sinned and deserve God’s curse.[iv]  Furthermore, the explicit testimony
that “the just shall live by faith”[v]
elminates the possibility that life comes from the law, for the just are all
those who are justified by faith (3:11).[vi]  The law sets a different and contrary
standard—life for sinless obedience.[vii]  Christ took the curse of the law upon
Himself on the cross so that the Gentiles could be accepted by God and receive
salvation in all its aspects, inclusive of both justification and the promise
of the Spirit, through faith.[viii]

Paul’s use of Genesis 15:6 and Habakkuk 2:4 in
Galatians 3 emphasize the receipt of justification through faith alone rather
than the faithfulness and holiness that are the fruit of justifying faith.  As the Apostle demonstrates, the Old
Testament is clear—righteousness before God is the possession of all those who
believe, rather than a possession of those who merit salvation by works.  However, the faithfulness that is the
fruit of the union with Christ entered into at the moment of justification is
by no means excluded in Galatians. 
The promised Spirit, who sinners receive through faith alone at the
moment of their justification (3:14), will produce His fruit (5:16-26; 4:6) in
those who have received Him.  Those
justified by faith alone will be led by the Spirit (5:18) into a walk of
holiness that is characterized by love, joy, peace, longsuffering, and other
holy Spirit-produced acts, rather than the fleshly works that characterize
those who will not enter the kingdom but suffer damnation (5:19-23).  Faith will work by love (5:6).  Indeed, the entire Christian life is
lived by faith in the Son of God (2:20; cf. 5:5).  The Christian dispensation itself is the coming of faith
(3:23, 25).  Justification by faith
alone (2:16, 21) does not lead to a life of sin, because the believer is
legally dead to the law, crucified with Christ, and alive to God (2:17-20). As
is clear in Genesis and Habakkuk, 
Galatians affirms the twin truths that justification in the sight of God
is by grace through faith alone, based on the imputed righteousness of Christ
alone, and that faithfulness and holiness are the inevitable consequents
springing from true faith.  The
just shall live by faith, as Abraham did.