Everywhere in the New Testament baptism is a sign of several things. It is a sign of repentance. To repent is to change direction. Christians take their marching orders from a different leader. We walk resolutely that road which leads to the kingdom of God.
Baptism is a sign of faith.
Faith is keeping company with Jesus Christ. Living unashamedly in his company, we share his identity. We are publicly known as those who know him and love him and obey him. It means that the repentance and faith and service we exercise, we exercise inasmuch as God's own Spirit, God himself, has touched us and moved us and constrained us. Baptism means this.