What exactly is the "Apostles' Doctrine"?

Question

Acts 2:42 "And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers."

My question is what exactly was the "Apostles' Doctrine" according to the Apostles who were present during the moment when the Holy Spirit filled the house?

Answer 1

Peter, Paul, and John provide some rather concise answers:

1 Corinthians 15:1, 3-4

1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;

3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;

4 And that he was buried, and that he arose again the third day according to the scriptures:

Acts 2:37-38

37 Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?

38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.

(note that this was preached right before the passage in question)

Acts 4:10, 12

10 Be it known unto you all, and to all the people of Israel, that by the name of Jesus Christ of Nazareth, whom ye crucified, whom God raised from the dead, even by him doth this man stand here before you whole.

12 Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.

John 20:31

But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.

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Their focus is on what Jesus did, and the process by which His sacrifice is made fully efficacious in our lives (belief on Him, repentance, baptism, receiving the Holy Ghost, etc.)

Conclusion

The apostles' doctrine = the atonement of Jesus Christ and how we use it.

Answer 2

The "Apostles' Doctrine" is what the apostles kept repeating. This was summarized in what is believed to be the first creed as recorded in 1 Cor 15:3-7 -

For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that He was buried, that He was raised on the third day according to the Scriptures, and that He appeared to Cephas and then to the Twelve. After that, He appeared to more than five hundred brothers at once, most of whom are still living, though some have fallen asleep. Then He appeared to James, then to all the apostles.

There are many other instances of this:

• Acts 2:32 - God has raised this Jesus to life, to which we are all witnesses. Exalted, then, to the right hand of God, He has received from the Father the promised Holy Spirit and has poured out what you now see and hear.
• Acts 4:10-12 - then let this be known to all of you and to all the people of Israel: It is by the name of Jesus Christ of Nazareth, whom you crucified but whom God raised from the dead, that this man stands before you healed. This Jesus is ‘the stone you builders rejected, which has become the cornerstone.’ Salvation exists in no one else, for there is no other name under heaven given to men by which we must be saved.”
• Acts 5:29-32 - But Peter and the other apostles replied, “We must obey God rather than men. The God of our fathers raised up Jesus, whom you had killed by hanging Him on a tree. God exalted Him to His right hand as Prince and Savior, in order to grant repentance and forgiveness of sins to Israel. We are witnesses of these things, and so is the Holy Spirit, whom God has given to those who obey Him.
• Acts 10:39-41 - We are witnesses of all that He did, both in the land of the Jews and in Jerusalem. And although they put Him to death by hanging Him on a tree, God raised Him up on the third day and caused Him to be seen— not by all the people, but by the witnesses God had chosen beforehand, by us who ate and drank with Him after He rose from the dead.
• Acts 13:28-33 - And though they found no ground for a death sentence, they asked Pilate to have Him executed. When they had carried out all that was written about Him, they took Him down from the tree and laid Him in a tomb. But God raised Him from the dead, and for many days He was seen by those who had accompanied Him from Galilee to Jerusalem. They are now His witnesses to our people. And now we proclaim to you the good news: What God promised our fathers He has fulfilled for us, their children, by raising up Jesus. As it is written in the second Psalm: ‘You are My Son; today I have become Your Father.’
• Acts 17:30, 31 - Although God overlooked the ignorance of earlier times, He now commands all people everywhere to repent. For He has set a day when He will judge the world with justice by the Man He has appointed. He has given proof of this to everyone by raising Him from the dead.”
• Rom 1:2-4 - the gospel He promised beforehand through His prophets in the Holy Scriptures, regarding His Son, who was a descendant of David according to the flesh, and who through the Spirit of holiness was declared with power to be the Son of God by His resurrection from the dead: Jesus Christ our Lord.
• Rom 4:24, 25 - but also for us, to whom righteousness will be credited—for us who believe in Him who raised Jesus our Lord from the dead. He was delivered over to death for our trespasses and was raised to life for our justification.
• Rom 8:11 - And if the Spirit of Him who raised Jesus from the dead is living in you, He who raised Christ Jesus from the dead will also give life to your mortal bodies through His Spirit, who lives in you.
• Rom 10:8-10 - But what does it say? “The word is near you; it is in your mouth and in your heart,” that is, the word of faith we are proclaiming: that if you confess with your mouth, “Jesus is Lord,” and believe in your heart that God raised Him from the dead, you will be saved. For with your heart you believe and are justified, and with your mouth you confess and are saved.
• 1 Cor 6:14 - By His power God raised the Lord from the dead, and He will raise us also.
• 1 Cor 4:13-15 - And in keeping with what is written: “I believed, therefore I have spoken,” we who have the same spirit of faith also believe and therefore speak, knowing that the One who raised the Lord Jesus will also raise us with Jesus and present us with you in His presence. All this is for your benefit, so that the grace that is extending to more and more people may overflow in thanksgiving, to the glory of God.
• 1 Thess 1:10 - For they themselves report what kind of welcome you gave us, and how you turned to God from idols to serve the living and true God and to await His Son from heaven, whom He raised from the dead—Jesus our deliverer from the coming wrath.
• 2 Tim 2:8 - Remember Jesus Christ, raised from the dead, descended from David, as proclaimed by my gospel
• Heb 13:20, 21 - Now may the God of peace, who through the blood of the eternal covenant brought back from the dead our Lord Jesus, that great Shepherd of the sheep, equip you with every good thing to do His will. And may He accomplish in us what is pleasing in His sight through Jesus Christ, to whom be glory forever and ever. Amen.
• 1 Peter 1:3-5 - Blessed be the God and Father of our Lord Jesus Christ! By His great mercy He has given us new birth into a living hope through the resurrection of Jesus Christ from the dead, and into an inheritance that is imperishable, undefiled, and unfading, reserved in heaven for you, who through faith are shielded by God’s power for the salvation that is ready to be revealed in the last time.

... and so forth. The constant theme of the apostles' teaching and gospel was the good news that

1. Jesus died for our sins
2. Jesus was buried and in the tomb
3. Jesus was resurrected from the dead
4. Jesus is our Lord and the sum and substance of our faith to be resurrected in the last day

Answer 3

While Paul eventually would be called and chosen later on in Acts, and he would at that point likewise begin to preach and teach as an apostle of God the Father and the Lord Jesus Christ, I think to best answer the original question regarding exactly what the Apostles Doctrine was, as of Acts 2:42, it is important to stick with the Twelve before moving on to anything Paul said or wrote.

From Acts 1:1-4 (KJV), we know the following:

1 The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach, 2 Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen: 3 To whom also he shewed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God: 4 And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me.

Here, we see that Jesus through the Holy Spirit gave His apostles commandments. We also see that prior to those commands being given in and through the Holy Spirit, Jesus spent forty days with them speaking with them things pertaining to the Kingdom of God.

In the Gospels, Jesus frequently spoke to them the same things, so I suspect the things Jesus said to them during that forty day timeframe were reminders and summaries of the things He had already said over the course of His pre-Calvary ministry.

We can therefore see that the doctrine the Apostles preached and taught was not really theirs, per se, but rather, was the doctrine taught to them by the Lord, i.e. the things pertaining to the Kingdom of God (See also, e.g., Acts 13:12).

Interestingly, in John 7:16, Jesus said "My doctrine is not mine, but his that sent me."

This shows that, according to Christ, the doctrine He taught and espoused wasn't really even His, but rather, belonged to and came from the Father, i.e. God Himself.

This makes sense in light of John 13:20,

Verily, verily, I say unto you, He that receiveth whomsoever I send receiveth me; and he that receiveth me receiveth him that sent me.

There is an unbroken doctrinal line descending first from God the Father, to His Son, the Lord Jesus of Nazareth, and then, to the Twelve, then eventually to Paul of Tarsus and the other apostles and elders of the 1st Century Church.

As regards specifics, I recommend looking at Hebrews 6:1-3,

1 Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God, 2 Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment. 3 And this will we do, if God permit.

The above passage shows what the principles of the doctrine of Christ are, that is, what the original foundation that was laid, was:

1. Repentance from dead works
2. Faith toward God
3. Baptisms
4. Laying on of hands
5. Resurrection from the dead
6. Eternal judgment
7. Going on to perfection

These seven things seem to encapsulate the core teachings of the apostles.