Our testimony is what we know to be true from our own personal experience.
As we act in faith in Jesus Christ, following the whisperings of the Spirit, we come to know for ourselves what happens when one puts his trust in the Lord. The more obedient to the Lord we are, the more he blesses us. As he blesses us, we come to know for ourselves that the promises of the message of the Gospel of Jesus Christ are true. This knowledge is our testimony of the gospel. It grows in strength as we continue in obedience to the Lord. It decreases as we return to evil ways.
Having a testimony is no guarantee that anyone will continue faithful. Our agency allows us to choose as we wish. But no one can live all of the gospel without a strong testimony. The sacrifices are too great to perform unless we know for ourselves that we are doing the will of the Lord.