How did they differ from Protestants
[How did they differ from Catholics
How did they differ from Eastern Orthodox
How did they differ from Oriental Orthodox
Thank you and God bless
Unlike Protestants, they did not believe in any of the Five Solas, and there was no Iconoclasm among them, and the Eucharist was held in high reverence which among Protestants we really only see among Anglo Catholics and Lutherans.
Unlike Roman Catholics, the early Church had no concept of Papal Supremacy, Papal Infallibility (which was dogmatized at the First Vatican Council n the 1860s and caused a schism with the Old Catholics), Purgatory or Indulgences. That said, Rome has had celibate priests for its recorded history, and indeed made an effort through its legates to have celibacy for priests included in the canons of the Council of Nicaea, but the bishops voted this down.
The only major difference compared to the Eastern and Oriental Orthodox is that some local churches were permitted to celebrate the Pascha on the 14th of Nissan; Quartodecimianism was not prohibited until the Council of Nicaea. Additionally, the Syriac speaking Christians converted by St. Thomas the Apostle got vernacular Bibles in the fourth century, the Peshitta, and the Copts got vernacular Bibles in both the Sahidic and Bohairic dialects starting in the fifth century.
Also, the liturgical music has become better, for example, a few years prior to his martyrdom, I would say around 100 AD, St. Ignatius of Antioch had a dream that inspired the development of antiphonal music. This led to the development of a diverse array of different types of hymns in the various Orthodox churches, such as the Kontakion, Canon and Troparion in the Byzantine Rite, and the Qanone and the metrical homily in the Syriac Rite, and the eight tone system of chant (Octoechos) in both the Byzantine and Syriac Rite, which was also implemented in Gregorian Chant and other Orthodox rites, and in the Coptic church, a system of chant called Tasbeha, which has named rather than numbered tones.
Basically the Eastern and Oriental Orthodox churches, and some Eastern Catholic churches, are as close as you can get to the Early Church. In the case of the Roman church, considering that prior to the late sixth century under Pope St. Gregory, they used monotone hymns exclusively (except in Milan, which began developing its own liturgy under St. Ambrose, and other places under the Gallican liturgical rite, which was later suppressed by Charlemagne in favor of the Roman rite), their liturgy improved very dramatically. However, Pope Gregory was one of the last Bishops of Rome who upheld the extreme conservatism which characterized the church in Rome previously (but not other Western churches which were part of the Roman Patriarchate). Rome never had an Arian or Nestorian or Iconoclast Pope (although it did have a Monothelite Pope, Honorius I, who is the only Pope who supported a doctrine that would later be anathematized by one of the Seven Ecumenical Councils).