As is shown in the above mystery verses, the "mystery of godliness" (1 Tim 3:16) and the "mystery of Christ in you" (Col 1:27) was not clearly understood or revealed to the Old Covenant Believers.
Isaiah 48:12-16 comes as close to revealing the mystery of the Triune One God as any place in the Old Covenant,
12 "Listen to Me, O Jacob, even Israel whom I called; I am He, I am the first, I am also the last. (Rev 1:17)
13 "Surely My hand founded the earth, And My right hand spread out the heavens; When I call to them, they stand together. (John 1:3)
14 "Assemble, all of you, and listen! Who among them has declared these things? The LORD loves him; he will carry out His good pleasure on Babylon, And His arm will be against the Chaldeans.
15 "I, even I, have spoken; indeed I have called him, I have brought him, and He will make his ways successful.
16 "Come near to Me, listen to this: From the first I have not spoken in secret, From the time it took place, I was there. And now the (1) Lord GOD has sent (2) Me, and His (3) Spirit."
Nevertheless, Old Covenant Believers were still redeemed by looking forward to the coming Messiah, the Lamb of God who saves us from our sins (John 1:29).
Jesus' disciples certainly were given clear understanding of who Jesus was in these verses in John 14:9-11,
9 Jesus said to him, "Have I been so long with you, and yet you have not come to know Me, Philip? He who has seen Me has seen the Father; how can you say, `Show us the Father'?
10 "Do you not believe that I am in the Father, and the Father is in Me? The words that I say to you I do not speak on My own initiative, but the Father abiding in Me does His works.
11 "Believe Me that I am in the Father and the Father is in Me; otherwise believe because of the works themselves.
New Covenant Believers are saved from our sins by applying the truth of Acts 2:38-39,
Peter said to them, "Repent, and each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the Holy Spirit. For the promise is for you and your children and for all who are far off, as many as the Lord our God will call to Himself."
When unbelievers reject the "mystery of godliness" that has been revealed by the Lord Jesus Christ to His Apostles beginning with Pentecost, then all unbelievers have left are meaningless arguments and dead religions that cannot save anyone from their sins or give eternal life to anyone.
Jesus declared this in John 11:25-26,
25 Jesus said to Martha, "I am the resurrection and the life; he who believes in Me will live even if he dies,
26 and everyone who lives and believes in Me will never die. Do you believe this?"
Jesus' question to Martha still applies to every man and woman: Do you believe this?