first-born (v.18) — here, primarily precedence in time (in other places, and somewhat here, it indicates supremacy in rank). He was the first to rise from the dead into true resurrected life (not to die again). He was the first to rise of His own power.
In the words, “the firstborn of the dead,” Paul shows how Christ is the beginning of the new spiritual life in the Church, by His resurrection. He comes forth from among the dead as the first-born issues from the womb. Compare Acts 2:24, “having loosed the pains of death” where the Greek is odinas, “birth-throes.” — Wuest, page 186.
so that (v.18) — By His resurrection, He entered a wider and more significant sovereignty. Who was declared the Son of God with power by the resurrection from the dead, according to the Spirit of holiness, Jesus Christ our Lord (Romans 1:4).
The contrast in verbs must not be overlooked. Christ is first in reference to all creation (v.17); His resurrection made it possible for Him to become first with respect to the church (cf. Philippians 2:9-11). What He “is” eternally to finite existence at large, He “becomes” actually to His new creation in His finished and victorious sacrifice and risen life. — Vaughan, page42
everything (v.18) — in the Greek, all inclusive — all things
It is significant that Paul says “all” the fullness dwells in Christ. The Gnostic heretics parceled out deity among the many spirit beings which they thought of as filling the space between God and the world. They looked upon these powers as intermediaries and taught that any communication between God and the world had to pass through them. They probably included Christ among these supernatural powers, admitting that He was of heavenly origin and that God was in some sense present in Him. He was, however, only one aspect of the divine nature and in Himself was not sufficient for all the needs of men. Paul, in contrast, declares that deity is not distributed among a hierarchy of powers; Christ is not just one of many divine beings. He is the one mediator between God and the world; and all, not part, of the attributes and activities of God are centered in Him. — Vaughan, pages 44-45.
the Father (v.19) — not in the original. The context indicates “God,” which includes the whole Trinity.
fullness (v.19) — the sum total of the divine power and attributes