The message of the angel began with asking why Alma the younger persecuted the church. The angel noted that the church was truly Jehovah’s church. That meant that fighting against the church was fighting directly against Jehovah.
Now the angel admonishes Alma the younger to remember his father’s experience of bondage in the land of Helam. Jehovah freed the people. By implication, Alma the younger is also under a form of bondage. He is also captive of spiritual Lamanites, rather than physical ones, but the effect is the same. He is to remember the deliverance from Helam because it can presage his own deliverance from spiritual bondage through Jehovah’s mercy and power.