"How much more shall the blood of Christ, who through the eternal Spirit offered Himself without spot to God, purge your conscience from dead works to serve the living God?" (Hebrews 9:14).
There are a number of significant references to the work of all three Persons of the Trinity in the great work of salvation. Note the implicit reference to the Trinity in our text.
" . . . the blood of Christ . . . through the eternal Spirit offered . . . to God." There is also a beautiful Trinitarian implication in Ephesians 2:18. "For through |Christ| we both have access by one Spirit unto the Father."
The promise of Christ to send the Holy Spirit is a high point of the Gospel of John. "I will pray the Father," said the Lord Jesus, "and He shall give you another Comforter" (Greek, parakletos, meaning "one called alongside"), "that He may abide with you for ever" (John 14:16). "But the Comforter, which is the Holy Ghost, whom the Father will send in my name, He shall teach you all things" (John 14:26). "But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, He shall testify of me" (John 15:26).
It seems very clear from such Scriptures that all three -- Father, Son, Holy Spirit -- are each distinct persons. Yet that the three together are one God is also clear from the fact that they are identified by name as One. Converts are to be baptized "in the name |note the singular -- one Name| of the Father, and of the Son, and of the Holy Ghost" (Matthew 28:19). Note also the benediction formula. "The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all. Amen" (II Corinthians 13:14). What we cannot fully understand in our minds of this wonderful triune Godhead, we can understand and believe with our hearts. HMM