Q. What is the Mystery mentioned in the following verses? Romans 16:25; 1 Cor. 2:7-8; Ephesians 3:2-5; 3:9-11; Col. 1:25-28; 4:3-4.
The "mystery" is simply something that in ages past has not been revealed. Read and study the passages below (and listen to the audio of this answer) and you will see that the "mystery" has to do with the plan of salvation God has had in mind for all mankind.
1 Cor. 2:7-8 NAS95
7* but we speak God's wisdom in a mystery, the hidden wisdom which God predestined before the ages to our glory; 8* the wisdom which none of the rulers of this age has understood; for if they had understood it they would not have crucified the Lord of glory;
Ephesians 3:2-5 NAS95
1* For this reason I, Paul, the prisoner of Christ Jesus for the sake of you Gentiles– 2* if indeed you have heard of the stewardship of God's grace which was given to me for you; 3* that by revelation there was made known to me the mystery, as I wrote before in brief. 4* By referring to this, when you read you can understand my insight into the mystery of Christ, 5* which in other generations was not made known to the sons of men, as it has now been revealed to His holy apostles and prophets in the Spirit; 6* to be specific, that the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise in Christ Jesus through the gospel, 7* of which I was made a minister, according to the gift of God's grace which was given to me according to the working of His power. 8* To me, the very least of all saints, this grace was given, to preach to the Gentiles the unfathomable riches of Christ, 9* and to bring to light what is the administration of the mystery which for ages has been hidden in God who created all things; 10* so that the manifold wisdom of God might now be made known through the church to the rulers and the authorities in the heavenly places. 11* This was in accordance with the eternal purpose which He carried out in Christ Jesus our Lord, 12* in whom we have boldness and confident access through faith in Him.
Col. 1:25-28 NAS95
25* Of this church I was made a minister according to the stewardship from God bestowed on me for your benefit, so that I might fully carry out the preaching of the word of God, 26* that is, the mystery which has been hidden from the past ages and generations, but has now been manifested to His saints, 27* to whom God willed to make known what is the riches of the glory of this mystery among the Gentiles, which is Christ in you, the hope of glory. 28 We proclaim Him, admonishing every man and teaching every man with all wisdom, so that we may present every man complete in Christ.
Col. 4:3-4 NAS95
3* praying at the same time for us as well, that God will open up to us a door for the word, so that we may speak forth the mystery of Christ, for which I have also been imprisoned; 4* that I may make it clear in the way I ought to speak.
Romans 16:25-26 NAS95
25* Now to Him who is able to establish you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery which has been kept secret for long ages past, 26* but now is manifested, and by the Scriptures of the prophets, according to the commandment of the eternal God, has been made known to all the nations, leading to obedience of faith;
1 Pet 1:10-12
10* As to this salvation, the prophets who prophesied of the grace that would come to you made careful searches and inquiries, 11* seeking to know what person or time the Spirit of Christ within them was indicating as He predicted the sufferings of Christ and the glories to follow. 12* It was revealed to them that they were not serving themselves, but you, in these things which now have been announced to you through those who preached the gospel to you by the Holy Spirit sent from heaven–things into which angels long to look