The specific term, “merits” applied to the activity of the saints, appears in the Liturgy of the Church by the 4th century. We certainly see it in the Canon of the Mass perhaps earlier than this. In various collects and prayers it appears at least by the 4th century, and continuously thereafter.
Indulgences
I cannot attempt here to investigate the entire Roman Catholic doctrine of Indulgences. I only wish to comment briefly on the doctrine as it affects the practice of devotions. It does, indeed have an effect because the trend since the 17th or 18th centuries has been to grant indulgences mostly for performing devotions. Therefore, the practice of devotions has become linked in the popular mind with the gaining of indulgences.