A filled mind (1:9) results in good works (1:10) and divine strength (1:11). Paul now adds to this a third result: thanksgiving that is continual[1] and joyful.[2] Such gratitude is apropos for the recipients of the great blessings of God.[3]
Qualification (12): God made us fit[4] for something that is beyond our reach: "the inheritance of the saints." This is the possession of heaven[5] fit only for holy people.[6] Paul then adds another descriptor: "the saints in the light"![7] This last phrase highlights the saints' holiness even further. They are the antithesis of sinners in the "darkness" (1:13). All of this depicts our abject unworthiness to possess heaven. We were sinners in need of deliverance (1:13). We were in "the domain of darkness" (1:13). We needed "the forgiveness of sins" (1:14). All these obstacles put us far from the inheritance meant for sinless people with a perfectly holy life who need no pardon. Heaven was beyond our reach!
But notice, the Father didn't just dismiss this chasm as illusory. He instead dealt with our real disqualification. In Christ ("in whom" of 1:14), He forgave us (1:14), and so we became real "saints" (1:2)! In Christ, He truly erased our sins (2:13-14)! Thus, it is as holy people (through forgiveness of sins!) that we have the right to the saints' inheritance (Rev. 21:27). The Father qualified us!
[1] The participle for "giving thanks [eu-cha-ris-toun-tes (εὐχαριστοῦντες)]" is imperfective, hence, continual.
[2] NASB's "joyously giving thanks" is literally "with joy giving thanks." This is also "joyfully" in NIV and NET.
[3] Paul points out four works of God: qualification (1:12), and deliverance, transfer and redemption (1:13-14).
[4] NASB's "qualify" is the verb hi-ka-no-ō (ἱκανόω) which means "to make adequate/fit" for a task.
[5] "Inheritance [klē-ros (κλῆρος)]" emphasizes not the act of bequeathal but the thing given, the possession.
[6] "Saint" is the word "holy [ha-gi-os (ἅγιος)]." This is "holy people." Rev. 21:8 shows heaven is not for sinners.
[7] NASB's "in Light" is literally "in the light" (NET, NKJV), contra "the darkness [articular in Greek]" (1:13).