here are a few: the doctrine of virgin Mary, infant baptism, only some christians are declared saints, only some christinas are declared as priests, bioshops are not allowed to marry, latin mass services until 1965, special/holy days, doctrines founded on human traditions, praying repetitive words using Rosary beads, prayer to icons, adoration and prayer of Mary, prayer to saints, prayer to angels, the mass.
The Virginity of Mary at the Birth of Christ is very explicit in St Luke's and St Matthew's Gospel.
Infant baptism is indicated by St Paul's account that entire households were baptised (Acts 16, Acts 18) - and when he speaks of baptism replacing circumcision, which was practiced on 8 day old males. Also, many of the direct disciples of the Apostles, like St Polycarp mention infant baptism being practiced.
Priesthood. St Peter says that all Christians are part of a royal priesthood, which offers spiritual sacrifices. This is true. The fact that some exercise the priesthood in a sacramental way does not diminish this. At the altar, those who have received the laying on of hands (ordination) offer the sacrifice of Christ - and the baptised present participate in that offering in a way that the unbaptised cannot. ie. the baptised indeed have a type of priesthood. The distinction is between the group called disciples in Scripture, and the group called apostles - only apostles were at the Last Supper, where the Mass was instituted, when Our Lord took bread and wine and made/gave them as his body and blood.
The celibacy if bishops and priests is disciplinary and not doctrinal. It is the choosing of men from that group who have renounced marriage, for the sake of the kingdom. Indeed there have been many married bishops and priests in the Church's history, but the Church preferred celibacy because the truth is the Scripture was bourne out in experience: those who tend to their wives neglect the things of God; and those who tend the things of God neglect their wives.
The Mass in Latin. If any one language should be preferred in the Mass it should be Aramaic, but the use of the language of Christ is not essential. Latin was the language of Rome - and so only the Roman Rite of the Catholic Church would use Latin. Indeed, it still does. But language is inconsequential - the meaning is important.
Special Holy Days. Our Lord rose from the dead on the first day of the week, Sunday. If any day is going to be considered holy, it is Sunday. Indeed, Our Lord observed the holy days of the Jewish calender and indeed his death/resurrection occurred coinciding with the Passover, and Pentecost coincided with the feast of the Jewish feast of tabernacles. The Scriptures are replete with references to gatherings on the first day of the week. The pattern of the New Testament feasts mirrors the pattern of the OT feasts, except where the OT feasts were an anticipation, the NT are a memorial/commemoration. "Do this in memory of me".
"Doctrines founded on human traditions" is not an example, but an accusation.
Praying repetitive words. The Scriptures exhort one to pray without ceasing. The Apostles are recorded as keeping the hours of prayer both at the Temple, even after the Ascension, and elsewhere. The hours of prayer were characterised by the reciting of psalms - repetitively. There are only 150 psalms and they were recited in whole each week. This practice of the Apostles is maintained even now, as all Catholic priests, bishops, and religious are obliged to pray the psalms every week. For those who were illiterate, or children, short versions of this kind of prayer emerged out of devotion. The shortest version of a 150-fold Scriptural form of prayer involved saying 10 times the words of St Gabriel to Mary, the words of St Elizabeth to Mary (short scriptural quotes) and then an Our Father. This gives you 15 lots of 10 - or 15 decades, which the modern rosary still has. The short quotes were then elaborated into a prayer form, in what is now called the Hail Mary. This form of prayer is a simple devotional form of prayer, derived from the desire to pray as the Apostles did (150 psalms) for those who do not have the time/ability to devote to them.
Icons are not prayed to. Rather, prayers are offered to the person the icon represents. St Paul, teaching about pagan idols makes two distinctions: an idol is nothing to fear because it is simply a piece of wood, or stone; but the idols can represent devils, or other evil beings. Likewise in terms of what a figure can represent, a crucifix is just a piece of wood, but it represents Christ at his Crucifixion. The jews were forbidden from having graven images, lest they worship them - and this makes sense because God, being a pure spirit, cannot be represented by any physical thing. Nevertheless, the Jews had cherubim atop the Ark of the covenant and the bronze serpent, which healed those bitten by snakes (still used as a symbol of the medical profession). But since God became man, (man who himself was made in the image of God) it is possible to represent God by some physical form - a crucifix. As for representation of the saints - saints are representative of Christ: "he who receives you, receives me", "whosoever gives you some water on my account", "when you did it to the least of these, you did it to me". So a saint can be honoured, as an extension of honouring Christ - and only in connection to Christ.
It is condemned as heretical by the Catholic Church to worship Mary. God alone is worshiped as God (latria) and the saints are honoured, as explained above in connection to Christ (dullia) to Mary, being the mother of Christ a higher honour is given than to the other saints (hyperdullia), she is honoured insofar has she became a dwelling place of the Holy Spirit and the Father and the Son. Moreover, when the infant St John the Baptist lept for joy in St Elizabeth's womb and St Elizabeth had called her blessed, that Mary prophesised that "henceforward all generations shall call me blessed, for the Almighty has done great things for me and Holy is His Name".
Prayer is defined as a loving conversation in Christ. That conversation can be with God, angels, or men. However, it is dangerous and to no good avail to converse with fallen angels, or the damned, so that prayers are only made with God, the angels and the saints. There are numerous examples of people speaking with Christ, with the Apostles, with the Blessed Virgin - indeed Mary interceded for the newly weds, and Christ performed his first public miracle at her prompting. Mary conversed with the Archangel Gabriel, and Tobias with St Raphael.
I think protestants probably have hang-ups with praying to saints and angels because they have jettisoned all worship of latria - ie. they got rid of the Mass; and so they cannot distinguish between worship and prayer.
The Mass is commanded by Our Lord at the Last Supper, for what the Mass is is nothing unless bread and wine are changed into the body and blood of Our Lord, as the new and eternal covenant.
