A phenomenon of great significance in the patristic period was the rise and gradual development of veneration for the saints, more particularly for the Blessed Virgin Mary. 

. . . Earliest in the field was the cult of martyrs . . . At first it took the form of the reverent preservation of their relics and the annual celebration of their ‘birthday’. From this it was a short step, since they were now with Christ in glory, to seekig their help and prayers, and in the third century evidence for the belief in their intercessory power accumulates. . . . By the middle of the same [4th] century, according to Cyril of Jerusalem, the patriarchs, prophets, apostles and martyrs were commemorated in the liturgy ‘so that by their prayers and intercessions God may receive our supplications’.

(J.N.D. Kelly, Early Christian Doctrines, San Francisco: Harper & Row, fifth revised edition, 1978, 490)

In the numerous memorial discourses of the fathers, the martyrs are loaded with eulogies, addressed as present, and besought for their protection. The universal tone of those productions is offensive to the Protestant taste, and can hardly be reconciled with evangelical ideas of the exclusive and all-sufficient mediation of Christ and of justification by pure grace without the merit of works. . . . The best church fathers, too, never separated the merits of the saints from the merits of Christ, but considered the former as flowing out of the latter.

(Philip Schaff, History of the Christian Church, Vol. III: Nicene and Post-Nicene Christianity: A.D. 311-600, Grand Rapids, Michigan: Eerdmans Publishing Company, 1974, from the revised fifth edition of 1910. chapter VII, section 84, 438)