Galatians 2:20-21

20 I have been crucified with Christ; it is no longer I who live, but Christ lives in me; and the life which I now live in the flesh I live by faith in the Son of God, who loved me and gave Himself for me.

21 I do not set aside the grace of God; for if righteousness comes through the law, then Christ died in vain.

I have been (v. 20) — the tense indicates “have been and still am.” A past completed action with continuous results.

Paul explained why and how he was dead to the law. Now (v.20), he explains why and how he lives.

The Judaizers shunned the reproach of the Cross; to them it was a “stumbling block,” (5:11; 6:12; 1 Corinthians 1:23). He [Paul] glorified in it and made it his own (6:14). Nor is it merely that he had “crucified” the grosser elements in his nature (5:24); he himself, his virtues and his vices, all that entered into and made the man, had been nailed to the Cross; henceforth he hoped nothing from the one, feared nothing from the other.

Christ, though He had fully discharged every obligation imposed by the law, endured the extreme penalty prescribed for “transgression and disobedience,” (Hebrews 2:2). When, therefore, a man believes on Christ, he acknowledges the judgment of God against sin to be just, and accepts the death of Christ as the execution of that judgment upon him for his own guilt. In thus believing the man becomes identified with Christ in His death, and since death nullifies all claims and all obligations, is “made dead to the law through the body of Christ,” (Romans 7:4) and ceases, finally, to be under the jurisdiction of the law — Vine, page 169

Christ lives in me (v.20) — by the Spirit (Romans 8:9-10)

the live which I now live (v.20) — by the power of the Spirit joined by association with Christ’s resurrection.

now (v.20) — since Paul’s salvation

in the flesh (v.20) — in his human body. Paul was still Paul, with his will, temperament, thoughts, hopes and fear — and sins. But through faith — and the faithfulness of Christ — he had new life.

I live by faith (v.20) — faithfulness, trustworthiness

Son of God (v.20) — “son” in the sense of eternal relationship and expression of character, not in the sense of deriving existence from.

The Bible refers to the love of God toward the world, but the love of the Son is only mentioned in relationship to believers.

gave Himself for me (v.20) — Christ died for each and He died for all.

set aside  (v.21) — make void, reject, nullify, do away with something established.

grace of God (v.21) — incompatible with works (Romans 11:6)

in vain (v.21) — without purpose or result

Paul was dead to his former life of legal righteousness and confidence in the law. Paul died to the law by dying with Christ who died under its penalty. The law’s demands were satisfied. Paul also died to self and his old life and now lived a Christ-centered life.