I Tim. 2:5-6  "For there is one God, and one mediator between God and men, the man Christ Jesus; Who gave himself a ransom for all, to be testified in due time."

Paul is impelled to preach, teach, and tell everyone he meets about the gospel.  He totally sums it up in this verse.

1.  We can't get to God without a mediator.  We are sinful.  God cannot look on sin.
2.  The mediator is Jesus.  He alone can approach the throne of God to speak for us.  Not saints, not Mary, not the preacher, not your Bible teacher, just Jesus.  One mediator.  Jesus.
3.  He gave himself.  If you think the Romans were responsible; or you think the Jews were responsible, you need to do a search on scriptural references.
      Two such references are Galatians 1:4 "Who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father:",  and Titus 2:14 "Who gave himself for us that he might redeem us from all iniquity, and purify unto himself a peculiar people, zealous of good works."
     Christ chose to die for our sins.  You remember in the garden when Jesus asked God if there was some other way for men to be saved.  He ended his prayer by saying "…not my will but thine be done."  And then he went to the cross.
4.  He was the ransom.  We had been kidnapped by sin and Jesus became the ransom payment for our release.  There was a trade.  His life for yours.
5.  And all of this would be testified in due time.  The day is coming when we will come up for judgement before God.  There will be testimony.  We will be guilty.  But if we have accepted Jesus as our mediator, He will defend and save us even though we are guilty.  He has paid the penalty for our release.