Articles 9-12 are distinguished from the preceding articles in that we do not believe in them. We believe in God the Father and we believe in Jesus Christ and we believe in the Holy Spirit. We believe One Holy Catholic and Apostolic Church. We believe the Communion of the Saints. Do not separate these articles either, as if they are just some sort of ad-on. There would be no Holy Catholic Church and no Communion of the Saints without the Holy Spirit.
This One Holy Catholic and Apostolic Church is not to be identified with a single institution or denomination. All who believe in the Trinitarian God that the Apostle's Creed speaks of are members of the One Holy Catholic and Apostolic Church. That doesn't mean that outward unity is irrelevant. Disunity hinders evangelism. Pray fervently for the unity of the Church and desire maximum outward unity that the world may believe (John 17). Do not seek unity at all costs. Unity must be found in genuine agreement in the truth.
The idea of a single institution containing all members of the One Holy Catholic and Apostolic Church seems very attractive, especially when you see the countless denominations. After all, there are no denominations in the Bible. We don't find Paul setting up his Lutheran church across the street from St. James Catholic Church. But within the institutions which do claim to be the One Holy Catholic and Apostolic Church there is a great deal of disunity. Nobody knows what kind of nutty thing the American Catholic Bishops might come up with next. Eastern Orthodoxy does a better job at creating the illusion of unity than most but they have their fair share of problems as well. Orthodox monks from various branches of the Orthodox church get in fist-fights in the Holy Land. People from different communions have trouble having their ordination recognized. Roman Catholics and Eastern Orthodox also use a number or loopholes in their own language and acknowledge that there are Christians outside of their own communions.
So we are left with the paradox of the One church that looks like anything but one. The Church is holy because she is washed in the blood of Christ. The Church is Catholic because she contains all believers from all over the world. The Church is Apostolic, not because of some questionable succession (although this is desirable) but because she maintains the Apostolic teachings.
The Creed goes on to say, "I believe the Communion of the Saints" or it could be translated "I believe the Communion in Holy Things." Perhaps more likely, it means both. All who are part of the One Holy Apostolic Catholic Church are part of the Communion of the Saints. You are part of a family and communion that is composed not only of believers who are now in your own church or now alive on the planet earth. You are part of a communion that includes more dead than living. You are part of a communion that includes every believer that ever lived.
You are part of the communion in holy things--the communion in the holy sacraments--especially the Sacrament of the Altar (Eucharist, Mass, Lord's Supper). You are part of the communion that partakes of the Lamb's body and blood. You are connected in communion to all believers because you are connected to the Lamb who shed His blood for you.
None of this should make you think that the visible church on earth is unimportant. The visible church on earth is where God brings us the Gospel and gives us the holy things. When we worship the Lamb in the local visible church, we join in worship with angels and archangels and all the company of heaven!