Gr. psuchē kai pneuma, here, “life and breath.” On psuchē see on Matt. 10:28, and on pneuma see on Luke 8:55. This and the expression “joints and marrow” are used figuratively. The idea of dividing between “soul and spirit” and between “joints and marrow” describes the sharpness of God’s “word.” The value of the figure is based on the idea that “life” and “breath” are, for practical purposes at least, inseparable.

Gr. merismos, “division,” “separation.”

 See Eph. 6:17; see on Rev. 1:16.

 The nature of this sharpness is explained in the remainder of v. 12. The “word” of God is living, powerful, and sharp—fully able to accomplish His beneficent purposes for mankind (see Isa. 46:10; 55:10, 11).

 Gr. energēs, “effective,” “active,” “powerful” (cf. on 1 Cor. 12:6). Our word “energy” is derived from energēs. There is power in God’s “word” to transform sinners into saints.

 That is, “living.” It takes a living and active force to create in man a new heart and to renew a right spirit within him (cf. Ps. 51:10). The “word” of God is living, and imparts life. Thus it was in the work of creation (Ps. 33:6, 9 and thus it is in the re-creation of the image of God in the soul of man. Christ, the incarnate Word, likewise has life in Himself (John 1:4, 12; 5:26). The “word of God” is the energizing force in conversion. The Christian is “born again, ... by the word of God, which liveth and abideth for ever” (1 Peter 1:23).

 Gr. logos (see on John 1:1). In the context the “word” here referred to is the “word” that was “preached” both to ancient Israel and to Christians (Heb. 4:2), particularly with respect to the invitation to enter into God’s “rest.” As v. 2 makes clear, this “word” is equated with the “gospel” message. In an extended sense the “word” of v. 12 may also be thought of as including all the writings of the Sacred Canon, inasmuch as the entire Bible is the “word” of God and sets forth the “gospel” of Jesus Christ.

 At first sight vs. 12 and 13 may appear to have no direct connection with the theme of chs. 3 and 4. However, as the word “for” indicates, there is a close, logical relationship. Verses 12 and 13 explain how to avoid falling in unbelief (v. 11) and how to cease from one’s “own works” (v. 10), and set forth the means God has provided to enable His children to enter into His “rest.”

Also, presumably, in the figure, practically inseparable.

 Gr. kritikos, “able to discern,” “able to judge,” “able to discriminate,” that is, possessed of the quality of discernment or discrimination. The English word “critic” is derived from kritikos. By a favorable response to the impression made upon the conscience by God’s “word” the sincere Christian avoids falling into “unbelief,” ceases from “his own works,” and enters into God’s “rest” (vs. 6, 10, 11; cf. ch. 3:10, 12).

Or, “thoughts and motives,” “thoughts and intentions” (RSV). Like a sharp blade separating “joints” from “marrow,” the clear principles of the “word of God” discern between good and evil thoughts, right and wrong motives.

That is, the mind.