Beginning/Firstborn (18b): Paul opens this section[1] by calling Christ "the beginning."[2] Given His creatorhood (1:16) and pre-existence (1:17), one expects this title. However, Paul is no longer speaking of the created order, but of redemption, Christ's victory over death.[3] He is the beginning as "the firstborn [pre-eminent one (cf. 1:15)] from the dead." Others physically rose from the dead, but none of them rose unto glory like Christ.[4] When Christ rose in glory, He took "the first place in everything," even as the victor over death.

Fullness (19): Paul gives the reason for Christ's primacy: "For it was the Father's good pleasure for all the fullness to dwell in Him." The Father wanted His incarnate Son to have all of His fullness.[5] His beloved Son must be fully supreme, whether authority (Matt. 28:18) or people (John 17:2) or judgment (John 5:22) or the nations (Ps. 2:8) or name (Phil. 2:9) or seat (Eph. 1:20-21).

Reconciliation (20): The Father's second good pleasure is the reconciliation of all things through Christ's sacrifice: "the blood of His cross." The Father made His Son the only path to peace with Him (John 14:6). Now, this peace is clearly with redeemable human beings (1:21-29), hence, Paul first speaks of those on the earth,[6] and then mentions those already in heaven. Either way, God desired to make that peace only possible through His incarnate Son.

[1] "And He is the beginning" is the second relative clause "who is the beginning" a la 1:15 "who is the image."

[2] Jesus affirms: "I am the Alpha and the Omega, the first and the last, the beginning and the end" (Rev. 22:13).

[3] As it was in 1:15, "the firstborn from the dead" defines the meaning of "the beginning" as an apposition.

[4] No one was raised unto glory before He rose in glory. Only after Him did anyone rise thus (Matt. 27:52-53).

[5] "All the fullness" is God's fullness (2:9), hence, "all his fullness" (NET) and "all the fullness of God" (ESV).

[6] In this context of redemption of people, "all things [not in Greek]" is better taken as "all peoples/nations."