All of humanity is called to believe in Jesus Christ alone for salvation. There is, after all, salvation in no other. When we believe in Christ, we place our weight on Him totally as the deliverer from our sins. We rely on Him; we depend on Him.
In believing in Christ we are to believe in the true Christ who is revealed in Scripture and Scripture alone. We are not to believe in concocted misrepresentations of the Christ who are only false Christs. The crucial implication of this is that we believe in the One who was incarnate, born of a virgin, delivered up to death on the cross of Calvary, shedding His blood for the forgiveness of the sins of all who believe. We believe in the One who was buried and rose again on the third day; ascended into heaven and will return again. If we have faith in any other Christ, we are not believing in the true Christ for salvation.
Further, our faith, as Christians, is in a real living being. When we connect with Christ, by faith, we connect with one who is animate with life. furthermore, It is through Him we are brought to the God of life.
Which brings me to the original question do we have faith in the blood of Christ? To which I answer that Scripture teaches that we are to have faith in the Christ who shed He blood on the cross and not faith in the cross itself. To emphasize the matter I ask the question in a slightly different way: Do we have faith in Jesus’ blood or in His cross? The answer is neither. Paul puts it like this when he says in Ephesians 1:7-8a In him we have redemption through his blood, the forgiveness of sins, in accordance with the riches of God’s grace that he lavished on us. Redemption is in Him who shed His blood and not in His blood itself.
Accordingly, we do not preach the crucifixion, but we preach the crucified One. Again we listen to Paul who says we preach Christ crucified: a stumbling-block to Jews and foolishness to Gentiles, but to those whom God has called, both Jews and Greeks, Christ the power of God and the wisdom of God (1 Cor. 1:23-24).
But what of Romans 3:25? There the KJV has the phrase “faith in his blood”. W.E Vine says this. “The rendering “faith in His blood” is incorrect. Faith is never said to be in the blood. Faith is imposed in a living person. Faith is the means of making the pardon ours; the blood is the means of its effect. The preposition of the original is instrumental. The phrase “by His blood” expresses the means of propitiation.”