Brother Silverhair.
Do you believe that Adam and Eve did not understand and see Christ and his kingdom in the sacrifice of the ram offered for them in Genesis 3:21.
How could David be described as a man after God's own heart and not know the gospel of the kingdom? The scriptures tell us because God gave him a new heart (like unto me). (Ezekiel 36:26-27) David was of the spiritual seedline of the prophecy that Moses gave in Deuteronomy 18:15 when he said, "The Lord thy God will raise up unto thee a Prophet, from the midst of thee, of thy brethren, LIKE UNTO ME (having the heart and spirit of God), unto him shall ye hearken." This was the command to Israel who the Lord would approve the lead them in the way of salvation. It is repeated in Acts 13:22.
If people can be saved without the knowledge of Christ, to whom are they hearkening?
If there is another way of salvation, you should be able to be a witness of it and tell us what it is.
Concerning the prophets and righteous men, 1 Peter 1:10 says, "Of which SALVATION the prophets have inquired and searched diligently, who prophesied of the grace that should come unto you. (Christ and his kingdom, THE GOSPEL) vs 10. Searching what or what matter of time the Spirit of Christ which WAS IN THEM did signify when IT DID TESTIFY/WITNESS beforehand, the sufferings OF CHRIST AND THE GLORY THAT SHOULD FOLLOW, 12 Unto whom IT WAS REVEALED that not unto themselves but unto us they did minister/witness the things which are now reported unto you by them that have preached THE GOSPEL unto you by the Holy Spirit sent down from heaven, which things the angel desire to look into.
The spirit of Christ can only testify of the Godhead of which those in Christ are witnesses.
The above had been and were witness of Christ and his glory and continues be glorified in the scriptural church.
Ephesians 3:21 states, "Unto him be GLORY in the church BY CHRIST JESUS throughout ages, world without end, Amen.
Colossians 1:16, "For BY HIM were all things created, that are in heaven and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him and for him (HIS GLORY)
Colossians 1:18 says, "And he is the head of the body, the church: who is the beginning, the firstborn from the dead, that in all things he might have the preeminence/GLORY."
John 17:22 "And the GLORY thou hast given me I have given them, that they may be one even as we are one."
Isaiah 54:17."...this is the heritage/inheritance of the servants of the Lord, and their righteousness is OF ME saith the Lord. This is the imputed righteousness of Christ. What/whose righteousness do these people saved by some other way belong to?
Since all things were created for him and his glory, how can something not created by him (false doctrine or way of salvation) glorify him?
How can Christ be glorified and have the preeminence in things that cannot give him the glory because they are "saved" some other way?
How can a thing "saved" give the glory of salvation (his sufferings and the glory that should follow) unto a way not created to glorify Christ, the way?
How can it (them) be a TRUE witness of the glory/righteousness/salvation of a Christ of which they have no knowledge?