According to Romans 3:25, does "faith is his blood" mean that a person must understand that Christ's died and shed His blood for their sins. Isn't simply trusting Christ as Savior enough?

There are some differences of opinion among Christian scholars when it comes to the answer to your question. In very general terms the Bible teaches that we must believe in the Lord Jesus Christ to be saved (John 1:12; 6:28-29; Acts 16:31). Romans 10:8-11 adds the element of our Lord's resurrection, which Paul indicates is an essential part of the gospel also in 1 Corinthians 15:1-4ff. Hebrews (chapters 9 & 10) and 1 Peter (1:18-19), among others, place much emphasis on the shed blood of Christ.

I think the answer to your question is that in order to be saved, one must surely know that he or she is a sinner, rightly condemned by God, and deserving of eternal punishment. They must realize that they cannot save themselves by any kind of good works, and that the only provision for their salvation is the death, burial, and resurrection of Jesus Christ on their behalf.

All of this will certainly not be fully grasped by a person (especially a child) at the moment of their conversion, but they surely should grow in their understanding of the work of Christ on their behalf over time. We must not forget that Jesus Himself said that we must come to Him "like a child" (Mark 10:15; Luke 18:17).

As I look at the disciples, I see that their faith in Jesus continued to grow throughout the years they were with Him, and especially after His death and the coming of the Holy Spirit. The shed blood of Christ is the heart of the gospel, so we should in no way minimize its importance. I would not agonize about what you did not know or understand as a child, but would rejoice in what you now know and believe as an adult, knowing that even your present knowledge and understanding is not perfect:

1 Corinthians 13:9-12 [The NET Bible]

9 For we know in part, and we prophesy in part, 10 but when what is perfect comes, the partial will be set aside. 11 When I was a child, I talked like a child, I thought like a child, I reasoned like a child. But when I became an adult, I set aside childish ways. 12 For now we see in a mirror indirectly, but then we will see face to face. Now I know in part, then I will know fully, just as I have been fully known.

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