“For there is one God and one mediator between God and mankind, the man Christ Jesus, who gave himself as a ransom for all people. This has now been witnessed to at the proper time.” 1 Timothy 2:5-6
“One God” is a central truth of the Bible. The only living God desires all (both Jews and Gentiles) to be saved.
“Mediator” is a concept derived from the ceremonial worship prescribed in the Old Testament. In the tabernacle and later in the temple, the priests mediated between God and the Israelites by offering animal sacrifices to atone for the sins of the people and by interceding to God for the nation. In their position as mediators, the priests were the only ones eligible to enter the Holy Place, the place where God had made His presence known.
The “one Mediator” is “the Man Christ Jesus”. There is one God from whom salvation is available. There is only one way to Him, through the Mediator, Christ Jesus, who has the full nature of God and the full nature of man. Jesus is both true God, begotten of the Father from eternity, and also true man, born of the Virgin Mary. As a true man, His work of mediation was carried out on behalf of all human beings.
“Ransom” is the price that must be paid to free captives (slaves). Jesus willingly gave His life as the ransom for all held captive by sin and death. The Greek word translated ransom is found only here in the NT. In Greek it is formed with a prefix that reinforces the idea of substitution. In other words, Christ substituted His life for ours. Our sins had separated us from God. Christ paid the penalty for our sins so that we could be reconciled to our Father.
“Witnessed to at the proper time” is Jesus’ act of self-giving, which testifies to the desire of the one God that all people be saved. Paul and the other apostles testified to Jesus’ mediating work, and their testimony continues through the time of the New Testament until Christ returns.