Transubstantiation was taught by Jesus, the Apostles, and ECFs?
Yes it is.
The Didache or “The Teaching of the Twelve Apostles” is a manuscript which was used by 2nd century bishops and priests for the instruction of catechumens. Many early Christian writers have referenced it making this document relatively easy to date.
“Let no one eat and drink of your Eucharist but those baptized in the name of the Lord; to this, too the saying of the Lord is applicable: ‘Do not give to dogs what is sacred’”.
-Ch. 9:5
“On the Lord’s own day, assemble in common to break bread and offer thanks; but first confess your sins, so that your sacrifice may be pure. However, no one quarreling with his brother may join your meeting until they are reconciled; your sacrifice must not be defiled. For here we have the saying of the Lord: ‘In every place and time offer me a pure sacrifice; for I am a mighty King, says the Lord; and my name spreads terror among the nations.’”
-Ch 14
St. Ignatius became the third bishop of Antioch, succeeding St. Evodius, who was the immediate successor of St. Peter. He heard St. John preach when he was a boy and knew St. Polycarp, Bishop of Smyrna. Seven of his letters written to various Christian communities have been preserved. Eventually, he received the martyr’s crown as he was thrown to wild beasts in the arena.
“Consider how contrary to the mind of God are the heterodox in regard to the grace of God which has come to us. They have no regard for charity, none for the widow, the orphan, the oppressed, none for the man in prison, the hungry or the thirsty. They abstain from the Eucharist and from prayer, because they do not admit that the Eucharist is the flesh of our Savior Jesus Christ, the flesh which suffered for our sins and which the Father, in His graciousness, raised from the dead.”
“Letter to the Smyrnaeans”, paragraph 6. circa 80-110 A.D.
“Come together in common, one and all without exception in charity, in one faith and in one Jesus Christ, who is of the race of David according to the flesh, the son of man, and the Son of God, so that with undivided mind you may obey the bishop and the priests, and break one Bread which is the medicine of immortality and the antidote against death, enabling us to live forever in Jesus Christ.”
-“Letter to the Ephesians”, paragraph 20, c. 80-110 A.D.
“I have no taste for the food that perishes nor for the pleasures of this life. I want the Bread of God which is the Flesh of Christ, who was the seed of David; and for drink I desire His Blood which is love that cannot be destroyed.”
-“Letter to the Romans”, paragraph 7, circa 80-110 A.D.
“Take care, then who belong to God and to Jesus Christ - they are with the bishop. And those who repent and come to the unity of the Church - they too shall be of God, and will be living according to Jesus Christ. Do not err, my brethren: if anyone follow a schismatic, he will not inherit the Kingdom of God. If any man walk about with strange doctrine, he cannot lie down with the passion. Take care, then, to use one Eucharist, so that whatever you do, you do according to God: for there is one Flesh of our Lord Jesus Christ, and one cup in the union of His Blood; one altar, as there is one bishop with the presbytery and my fellow servants, the deacons.”
-Epistle to the Philadelphians, 3:2-4:1, 110 A.D.
Justin Martyr was born a pagan but converted to Christianity after studying philosophy. He was a prolific writer and many Church scholars consider him the greatest apologist or defender of the faith from the 2nd century. He was beheaded with six of his companions some time between 163 and 167 A.D.
“This food we call the Eucharist, of which no one is allowed to partake except one who believes that the things we teach are true, and has received the washing for forgiveness of sins and for rebirth, and who lives as Christ handed down to us. For we do not receive these things as common bread or common drink; but as Jesus Christ our Savior being incarnate by God’s Word took flesh and blood for our salvation, so also we have been taught that the food consecrated by the Word of prayer which comes from him, from which our flesh and blood are nourished by transformation, is the flesh and blood of that incarnate Jesus.”
" First Apology", Ch. 66, inter A.D. 148-155.
“God has therefore announced in advance that all the sacrifices offered in His name, which Jesus Christ offered, that is, in the Eucharist of the Bread and of the Chalice, which are offered by us Christians in every part of the world, are pleasing to Him.”
“Dialogue with Trypho”, Ch. 117, circa 130-160 A.D.
Moreover, as I said before, concerning the sacrifices which you at that time offered, God speaks through Malachias, one of the twelve, as follows: ‘I have no pleasure in you, says the Lord; and I will not accept your sacrifices from your hands; for from the rising of the sun until its setting, my name has been glorified among the gentiles; and in every place incense is offered to my name, and a clean offering: for great is my name among the gentiles, says the Lord; but you profane it.’ It is of the sacrifices offered to Him in every place by us, the gentiles, that is, of the Bread of the Eucharist and likewise of the cup of the Eucharist, that He speaks at that time; and He says that we glorify His name, while you profane it."
-“Dialogue with Trypho”, [41: 8-10]
that one was pretty funny, and he brought up the Ten Commandments.