The tenth article of the Creed speaks about the Mystery of Baptism and about the remaining Mysteries.
Jesus Christ, sending His disciples out to preach, said, Go ye therefore, and teach all nations, baptizing them in the name of the Father and of the Son, and of the Holy Spirit, and added, Teaching them to observe all things whatsoever I have commanded you (Matt. 28:19-20). By this the Lord clearly stated that other mysteries had been established by Him.
By sacraments, or mysteries, are meant those holy acts through which the Holy Spirit mysteriously and invisibly confers Grace (the saving power of God) upon man.
The holy Orthodox Church has seven Mysteries: Baptism, Chrismation, Confession, Holy Communion, Marriage, Ordination, and Holy Unction.
The Symbol of Faith mentions only Baptism because that Mystery is the door into the Church of Christ. Only those who have been baptized can avail themselves of the other sacraments.
Moreover, at the time of the composition of the Creed, there were quarrels and doubts. For example, some thought that heretics who returned to the Church should be baptized a second time. The Ecumenical Council said that Baptism could be performed only one time for any given person. Therefore it is said — "I confess one Baptism." It is understood that this one Baptism must be performed in the true Church for it to be valid. This is true of all the Mysteries.