In the “Apostles' Creed” were they referring to themselves when they said “holy catholic church”?
The answer is, they are referring to the Mystical Body of Christ the Church that was born in the Upper Room. That's why it's called Holy because it was born from the Anointing of the Holy Spirit it's origin is Divine. The Church was first spoken by Jesus to Peter handing him the Keys to God's Kingdom.
*"And I tell you that you are Peter,[a] and on this rock I will build my church, and the gates of Hades[b] will not overcome it. 19 I will give you the keys of the kingdom of heaven; whatever you bind on earth will be[c] bound in heaven, and whatever you loose on earth will be[d] loosed in heaven.”(*Matthew16:18-19)
And from the Peter the Anointed Head of the Apostles and the Church of Christ that was yet to be seen was born from the All-Holy Woman the "Panagia".
St.Louie De Montfort one the Great Marian Saints had stated, If the Head of the Church Jesus Christ was born from the pure womb of the Woman by the overshadowing of the Holy Spirit, where does the Mystical Body the Church be born too? The Mystical Body cannot be born outside of Mary that would be a mongoloid pardon me from the term. The Mystical Body was born from the Sorrowful & Immaculate Heart of Mary.
721 *Mary, the all-holy ever-virgin Mother of God, is the masterwork of the mission of the Son and the Spirit in the fullness of time. For the first time in the plan of salvation and because his Spirit had prepared her, the Father found the dwelling place where his Son and his Spirit could dwell among men. In this sense the Church's Tradition has often read the most beautiful texts on wisdom in relation to Mary.101 Mary is acclaimed and represented in the liturgy as the "Seat of Wisdom.*"
The Logos and the Holy Spirit are both destined to use the flesh of the Woman to dwell among us and to indwelt in us.
The Head of the Church was born from the pure womb of the "All-holy" Theotokos and the Mystical Body of Jesus Christ the Church was born in the Sorrowful & Immaculate Heart of Mary and it happen in the Upper Room. This time not by Overshadowing anymore as Mary the Mother of the Church had teach all the Apostles and Disciples how to receive the Holy Spirit thru prayer and conversion. Please Book of Acts where Mary lead all the Apostles & Disciples in the spirit of prayer as one imploring the Holy Spirit to come.
750 To believe that the Church is "holy" and "catholic," and that she is "one" and "apostolic" (as the Nicene Creed adds), is inseparable from belief in God, the Father, the Son, and the Holy Spirit. In the Apostles' Creed we profess "one Holy Church" (Credo . . . Ecclesiam), and not to believe in the Church, so as not to confuse God with his works and to attribute clearly to God's goodness all the gifts he has bestowed on his Church.
If you belong outside of the Catholic Church don't worry as the Vatican II teaching defined all the Redeemed having goodwill belong to the People of God. So,really you are also part of the Mystical Body of Christ the Church. There is an element of Truth in some of the faith & religion outside the Catholic Church and by the inspiration of the Holy Spirit mercifully they will be drawn to the Fullness of Truth which only the One,Holy,Catholic, Apostolic Church possessed.As St.Paul the Apostle to the Gentiles had said;
"If I am delayed, you will know how people ought to conduct themselves in God’s household, which is the church of the living God, the pillar and foundation of the truth."
THE CHURCH - PEOPLE OF GOD
781 "At all times and in every race, anyone who fears God and does what is right has been acceptable to him. He has, however, willed to make men holy and save them, not as individuals without any bond or link between them, but rather to make them into a people who might acknowledge him and serve him in holiness. He therefore chose the Israelite race to be his own people and established a covenant with it. He gradually instructed this people. . . . All these things, however, happened as a preparation for and figure of that new and perfect covenant which was to be ratified in Christ . . . the New Covenant in his blood; he called together a race made up of Jews and Gentiles which would be one, not according to the flesh, but in the Spirit."201
To be acknowledge part of the People of God a follower of Christ must acknowledge Mary's role as the Mother of the Church. As Pope Francis had said, a christian cannot be orphan, Jesus gave us as the greatest gift At the Foot of the Cross. Jesus gave us Her own Mother to be our Mother in our christian journey.
"When Jesus therefore saw His mother, and the disciple whom He loved standing by, He said to His mother, “Woman, behold your son!” 27 Then He said to the disciple, “Behold your mother!” And from that hour that disciple took her to his own home."(John19:26-27)
The key word here is "Behold", why?
The Hebrew word YHWH is not a name but a description commanding Moses to "Behold" meaning to gaze the crucified man on the Cross, the image that seen by Moses. But, this profound mystery of God-Man Jesus Christ crucified on the Cross cannot be fathom alone.
A true christian need the Wisdom of the Mother when gazing At the Foot of the Cross.
I repeat, Apostles refers to the "Holy Catholic Church" because it was born of the same Woman which is acknowledge by the 2000 years Tradition both East & West as the "All-Holy or Panagia". The Upper Room is where the Mystical Body of Christ was born from the Sorrowful & Immaculate Heart of Mary not by "overshadowing but by the outpouring" of the Gifts of the Holy Spirit.
The question is, where in the bible that all the Redeemed was conceived by Mary in Her Immaculate Heart?
The answer is by meditating Genesis3:16 & Revelation12:2; a mother bear a son in sorrows...How much sorrows did the Theotokos bear in cooperating with Jesus redemptive mission in Her Sorrowful & Immaculate Heart?
Jesus commanded Mary to behold your son, the "son" pertains to all the redeemed.
Mary is the Mother of All the Redeemed by Her Second Fiat.