For example?
The Catholic church of 300 AD was much closer to Baptists of today than the Catholic church of today.
No one who has ever looked into the teachings of the early Church can seriously make this claim. To help you out, here are just a very few teachings/quotes from the Church of around 300 AD:
Efficacious Baptism
"Without baptism, salvation is attainable by none." (Tertullian, Baptism 1, A.D. 203).
Baptism and Confirmation
"In the Gospel our Lord Jesus Christ spoke with his divine voice, saying, ‘Except a man be born again of water and the Spirit, he cannot enter the kingdom of God’ [John 3:5]. This is the Spirit which from the beginning was borne over the waters; for neither can the Spirit operate without the water, nor the water without the Spirit. Certain people therefore interpret [this passage] for themselves wrongly, when they say that by imposition of the hand they receive the Holy Ghost, and are thus received, when it is manifest that they ought to be born again [initiated] in the Catholic Church by both sacraments." (Council of Carthage VII, Seventh Carthage, A.D. 256).
Physical Presence of Christ in the Eucharist
"We call this food Eucharist, and no one else is permitted to partake of it, except one who believes our teaching to be true and who has been washed in the washing which is for the remission of sins and for regeneration [i.e., has received baptism] and is thereby living as Christ enjoined. For not as common bread nor common drink do we receive these; but since Jesus Christ our Savior was made incarnate by the word of God and had both flesh and blood for our salvation, so too, as we have been taught, the food which has been made into the Eucharist by the Eucharistic prayer set down by him, and by the change of which our blood and flesh is nurtured, is both the flesh and the blood of that incarnated Jesus" (Justin Martyr, First Apology 66, A.D. 151).
"Do not, therefore, regard the bread and wine as simply that, for they are, according to the Master’s declaration, the body and blood of Christ. Even though the senses suggest to you the other, let faith make you firm. Do not judge in this matter by taste, but be fully assured by faith, not doubting that you have been deemed worthy of the body and blood of Christ" (Cyril of Jerusalem, Catechetical Discourses: Mystagogic 4:22:9, A.D. mid 300’s).
Priesthood and Forgiveness of Sins
"[The bishop conducting the ordination of the new bishop shall pray:] God and Father of our Lord Jesus Christ. . . . Pour forth now that power which comes from you, from your royal Spirit, which you gave to your beloved Son, Jesus Christ, and which he bestowed upon his holy apostles . . . and grant this your servant, whom you have chosen for the episcopate, [the power] to feed your holy flock and to serve without blame as your high priest, ministering night and day to propitiate unceasingly before your face and to offer to you the gifts of your holy Church, and by the Spirit of the high priesthood to have the authority to forgive sins, in accord with your command." (Hippolytus, Apostolic Tradition 3, A.D. 215).
Apostolic Tradition
"Papias [A.D. 120], who is now mentioned by us, affirms that he received the sayings of the apostles from those who accompanied them, and he, moreover, asserts that he heard in person Aristion and the presbyter John. Accordingly, he mentions them frequently by name, and in his writings gives their traditions [concerning Jesus]. . . . [There are] other passages of his in which he relates some miraculous deeds, stating that he acquired the knowledge of them from tradition" (Eusebius, Church History 3:39, A.D. 312).
Apostolic Succession
"[I]t is incumbent to obey the presbyters who are in the Church—those who, as I have shown, possess the succession from the apostles; those who, together with the succession of the episcopate, have received the infallible charism of truth, according to the good pleasure of the Father. But [it is also incumbent] to hold in suspicion others who depart from the primitive succession, and assemble themselves together in any place whatsoever, either as heretics of perverse minds, or as schismatics puffed up and self-pleasing, or again as hypocrites, acting thus for the sake of lucre and vainglory. For all these have fallen from the truth." (Irenaeus, Against Heresies 4:26:2, A.D. 189).
Mary: Ever-Virgin
"Let those, therefore, who deny that the Son is by nature from the Father and proper to his essence deny also that he took true human flesh from the ever-virgin Mary." (Athanasius, Discourses Against the Arians, 2:70 A.D. 360).
"We believe in one God, the Father almighty, maker of all things, both visible and invisible; and in one Lord Jesus Christ, the Son of God . . . who for us men and for our salvation came down and took flesh, that is, was born perfectly of the holy ever-virgin Mary by the Holy Spirit." (Epiphanius of Salamis, The Man Well-Anchored 120, A.D. 374).
Red Herring. What was the teaching of the church as a whole?
These are the teachings of individual teachers. As you know, various teachers differed with one another greatly over sundry items (still do). What became official doctrine? Officially, it is the creeds.
The teaching of the church as a whole is evidenced by the creeds at that time. Up until 400 and a little later there is very little (if anything- though a couple of things might need clarification) a Conservative Baptist (who from here on out will just be called "Baptist")would disagree with in the Creeds of that time. For example, which line of the Apostles Creed would a Baptist find offensive. If the words are understood- none.
Apostles' Creed
1. I believe in God, the Father Almighty, Creator of heaven and earth:
2. And in Jesus Christ, his only begotten Son, our Lord:
3. Who was conceived by the Holy Ghost, born of the Virgin Mary:
4. Suffered under Pontius Pilate; was crucified, dead and buried: He descended into hell:
5. The third day he rose again from the dead:
6. He ascended into heaven, and sits at the right hand of God the Father Almighty:
7. From thence he shall come to judge the quick and the dead:
8. I believe in the Holy Ghost:
9. I believe in the holy catholic church: the communion of saints:
10. The forgiveness of sins:
1l. The resurrection of the body:
12. And the life everlasting. Amen.
Also, individual beliefs often don't become official doctrine. For example, the belief in Limbo which many Catholics taught for centuries and which now may be categorically done away with. Some of the other practices such as latin Mass and certain penalties for various things have changed over the years. Doctrines have been added. Infant Baptism was not OFFICIAL until the 4th century. Prior to this, sprinkling was often a concession (if you couldn't immerse- you were allowed to sprinkle)
Even the formerly anathematized Protestants (see Council of Trent) are now considered "Separated Brethren" (Vatican II) or something to that affect whose baptism may be valid if done in the name of the Father, Son, and Holy Spirit. Though apparently Catholics believe we will do a little more purgatory time.
Official Baptist doctrine comes from Scripture. Baptists being independent, there is some lee-way on non essentials so you may see some Arminian Baptists for example and some some Calvinsts, some pre Trib Rapture folks and some Amillenial. Scripture is where most of the doctrines of the early Creeds came from (yes, after some debate over what Scripture means).
Catholic doctrine comes from Scripture plus Catholic tradition. The latter moves about fairly frequently depending upon what pressures are being placed upon the church at the time (such as will we see a softening on the stance on contraception? which some would consider a mortal sin?) or who happens to be in the Papal/Cardinal seats at the time.