Christians accept the Pentateuch and the rest of the Old Testament because Jesus, who is God in the flesh, accepts and teaches them (Matthew 4:4, 4:7, 4:10, 8:4; Mark 7:10, 10:2-3, 12:26; Luke 16:31, 20:37, 24:27, 24:44; John 1:45, 3:14, 5:45-46, 7:22).
Jesus was born into the tribe of Judah, a legal descendent of King David of Israel, as a fulfillment of Old Testament prophecy (1 Kings 9:5; Luke 1:32). If we were to reject the Old Testament as entirely false, we would have to claim that Jesus is not a fulfillment of Old Testament prophecy.
Yet He is, and He Himself tells us so repeatedly (Matthew 12:40, 16:4, 27:46 with Psalm 22:1; Luke 4:18-21, 11:30, 24:27, 24:44; John 1:45, 3:14, 5:45-46).
In the Torah, we read that sins are to be atoned for through the shedding of the blood of a spotless sacrifice (Leviticus 4:23-24). In the New Testament, it is this same divine command that explains for us the substitutionary atonement provided by Jesus the Messiah, our spotless sacrifice (Hebrews 7:26-27, 9:22; 1 Peter 1:19).
The Apostle Paul, a devout Jew and follower of Jesus Christ, explains that the Law of Moses in the Torah was given to men to display their sins to them, to show us our sinful imperfections and our need for divine assistance and mercy and grace (Romans 3:20, 7:24). He goes on to explain that we find this very mercy and grace in the person and sacrifice of Jesus (Romans 6:23, 10:9).
If Christians were to reject the Torah, we would also have to reject Christ and the New Testament. We accept the Torah because we accept Christ who not only teaches it but also fulfills it.