Jesus last command to his disciples in Acts was to be His witnesses. The reason is that the church grows by a person sharing their faith another person. The church grows by word of mouth. We refer to this as Christian witness, sharing faith with another person that they might come to trust in Jesus as Lord and Savior.
In the courtroom, what a witness testifies to is what they believe they saw or heard. Christian witness is not about what we saw, but what God has revealed to us in the person of Jesus Christ, His eternal Son. The foundation of Christian witness is not human reason, but revelation from God. It is God's truth revealed to us; Christian witness is not based on human suppositions.
Secondly, the content of our witness is not facts, but a person, Jesus Christ, the only Son of God. We testify to Christ's atoning work on the cross for the sin of the world. The sin that pervades the world is human self-will. My way, not God's way. Christ died on the cross as the Lamb of God to bear and take away our sin so we might have a renewed relationship with God the Father. Christ then calls us to be His witnesses to share His gospel with others that they might become members of His kingdom. The gospel spreads by one person sharing their faith with another person. The Holy Spirit quickens ears to hear and hearts to receive the message that God so loved the world He gave His only Son, Jesus Christ, to die on the cross for our sin that we might be reconciled to God. That is the foundation and content of our witness.