Apostles' and Nicene Creed. please Christians only?

The word 'catholic' does not mean the Roman Catholic Church or Orthodox Catholic or any member of a "Catholic" church; it simply means 'universal'. The term 'communion of saints' means the spiritual union of all Christians everywhere, living and dead, on earth or in heaven (or in purgatory, for those who believe in that, which I assume you don't.) In the Latin, the phrase is 'communio sanctorum', which could be translated 'communion of the holy' or 'communion of the sancitified' just as well as 'communion of saints'.