5. The third clause answers in parallelism to the first, the fourth to the second.
      I am the Lord's-- (Jer 50:5; 1Co 6:19, 20; 2Co 8:5).
      call himself by the name of Jacob--The Gentiles (as the result of the outpouring of the Holy Spirit on Israel, the Lord's "seed," first) shall join themselves to the children of Jacob, in order to worship their God (compare Isa 43:7; Ps 49:11). Or, "calls," that is, invokes and celebrates the name of Jacob, attaches himself to his nation and religion [MAURER], (Ps 24:6).
      subscribe . . . hand unto . . . Lord--in solemn and public covenant, pledging himself to God's service (compare Ne 9:38), before "witnesses" (Heb 12:1), after the manner of a civil contract (Jer 32:10, 12, 44). So the Christian in the sacraments [BARNES]. Literally, "shall fill his hand with letters (Ex 32:15; Eze 2:10) in honor of Jehovah"; or "shall write upon his hand, I am Jehovah's" (compare Isa 49:16; Re 13:16); alluding to the puncture with ink on the hand, whereby a soldier marked himself as bound to his commander; and whereby the Christians used to mark themselves with the name of Christ [LOWTH]. The former view is simpler.
      surname himself . . . Israel--MAURER and GESENIUS interpret this as the Hebrew sanctions, answering to their rendering of the parallel second clause, "calls blandly (speaks in honorable terms of) the name of Israel." Retaining English Version, we must, from the Hebrew understand it thus, "Surname himself by the honorable name of Israel" (Isa 45:4).

JFB.