Note that the mystery of the Gospel (of 1 Corinthians 15:1-4) is revealed through the prophecy (Romans 16:25-26) when the prophecy is understood properly (Luke 24:45-47, Acts 26:22-23).
That is, Christians can now understand how perfectly the Old Testament foretold the Gospel of the suffering and death of Christ for our sins, and His rising physically from the dead on the third day (Acts 26:22-23; 1 Corinthians 15:1-5, Luke 24:44-47). His suffering and death for our sins was foretold in Isaiah 53 (cf. Acts 8:32-35; 1 Peter 2:24). His crucifixion experience was foretold in Psalms 22 (cf. Matthew 27:46,35). His not remaining dead was foretold in Psalms 16:10 (cf. Acts 2:31). His rising from the dead on the third day was foretold in Hosea 6:2 (cf. 1 Corinthians 15:4, Luke 24:46, Colossians 2:12). The fact that Christ's New Covenant Gospel (Matthew 26:28) would go forth to save both Jews and Gentiles was foretold in Isaiah 49:6 and Isaiah 42:6 (cf. Acts 26:23b, Luke 24:47). For some other examples of how Jesus Christ fulfilled Old Testament scriptures at His first coming: He fulfilled Deuteronomy 18:15,18-19 (cf. Acts 3:22-24, Luke 24:44), and Zechariah 9:9 (cf. Matthew 21:4-5), and Psalms 118:22 (cf. Acts 4:11), and Isaiah 9:1-2 (cf. Matthew 4:12-16), and Psalms 110:4 (cf. Hebrews 6:20).
Note that they are the same thing (Colossians 1:18).