The Fathers of the Church
The Fathers of the Church have referred to the Church as "Mother Church" from as early as the second century AD. The following are some citations.

The Church as Mother in Visions 1-4 of The Shepherd of Hermas [second-century]

The second-century document, The Shepherd of Hermas, uses maternal imagery in the oldest layer of the text, Visions 1-4. Visions 1-4 present the church as an elderly woman whose appearance changes in response to the growing spiritual maturity of Hermas, the story's main character. In this text, the Mediterranean cultural values of honor and shame play an essential role in the construction of the symbol of the Church as Mother. The symbol of the Church as Mother rests primarily upon the instruction in moral goodness which Mother Church provides to her children. Since Mother Church "was created the first of all things" (Vis. 4.1.1), she becomes the major agent of salvation in Visions 1-4.

St. Clement of Alexandria: The Instructor of Children [1, 6. 41, 3] [ante 202 A.D.]:

When the loving an benevolent Father had rained down the Word, that Word then became the spiritual nourishment of those who had good sense. [42, 1] O mystic wonder! The Father of all is indeed one and the same everywhere; and one only is the Virgin Mother. I love to call her the Church. This Mother alone was without milk, because she alone did not become a wife. She is at once both Virgin and Mother: as a Virgin, undefiled; as a Mother, full of love.
Calling her children about her, she nourishes them with holy milk, that is with the Infant Word. . . . The Word is everything to a child: both Father and Mother, both Instructor and Nurse. "Eat My Flesh," He says, "and drink My Blood ." The Lord supplies us with these intimate nutriments. He delivers over His Flesh, and pours out His Blood; and nothing is lacking for growth of His children. O incredible mystery!

St. Cyprian of Carthage (circa 251 - 256): On the Unity of the catholic Church,
Chapter 5:
Thus too the Church bathed in the light of the Lord projects its rays over the whole world, yet there is one light which is diffused everywhere, and the unity of the body is not separated. She extends her branches over the whole earth in fruitful abundance; she extends her richly flowing streams far and wide; yet her head is one, and her source is one, and she is the one mother copious in the results of her fruitfulness. By her womb we are born; by her milk we are nourished; by her spirit we are animated.
Chapter 6:
The spouse of Christ cannot be defiled; she is uncorrupted and chaste. She knows one home, with chaste modesty she guards the sanctity of one couch. She keeps us for God; she assigns the children whom she has created to the kingdom. Whoever is separated from the Church and is joined with an adulteress is separated from the promises of the Church, nor will he who has abandoned the Church arrive at the rewards of Christ. He is a stranger; he is profane; he is an enemy. He cannot have God as a father who does not have the Church as a mother.

Chapter 23:
Indeed, I desire, most beloved brethren, and I likewise advise and entreat, that, if it can be done, no one of the brethren perish, and that our rejoicing Mother enclose in her bosom one body of people in agreement.

St. Cyril of Jerusalem: Catechetical Lectures XVIII, 26 (circa 350)

But since the word Ecclesia is applied to different things (as also it is written of the multitude in the theatre of the Ephesians, And when he had thus spoken, he dismissed the Assembly ), and since one might properly and truly say that there is a Church of evil doers, I mean the meetings of the heretics, the Marcionists and Manichees, and the rest, for this cause the Faith has securely delivered to thee now the Article, "And in one Holy Catholic Church;" that thou mayest avoid their wretched meetings, and ever abide with the Holy Church Catholic in which thou wast regenerated. And if ever thou art sojourning in cities, inquire not simply where the Lord's House is (for the other sects of the profane also attempt to call their own dens houses of the Lord), nor merely where the Church is, but where is the Catholic Church. For this is the peculiar name of this Holy Church, the mother of us all, which is the spouse of our Lord Jesus Christ, the Only-begotten Son of God (for it is written, As Christ also loved the Church and gave Himself for it , and all the rest,) and is a figure and copy of Jerusalem which is above, which is free, and the mother of us all ; which before was barren, but now has many children.

St. Augustine (354 - 430): Commentary on Psalm 88, 14:

Unflaggingly, let us love the Lord our God and let us love his Church. Let us love Him as the Lord and the Church as his handmaid. No one can offend the one and still be pleasing to the other. What does it avail you if you do not directly offend the Father but do offend the mother?