Core Teachings of Jesus:

Initial Message (acc. to Mark): "This is the time of fulfillment. The kingdom of God is at hand. Repent, and believe in the gospel." (Mark 1:15)
The Golden Rule: "Do to others whatever you would have them do to you." (Matt 7:12; par. Luke 6:31)
The Greatest Commandment: "You shall love the Lord your God with all your heart, with all your soul, with all your mind, and with all your strength... You shall love your neighbor as yourself." (Mark 12:30-31; par. Matt 22:37-39, Luke 10:27; citing Deut 6:4-5 & Lev 19:18)
Love of Enemies: "Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you." (Luke 6:27-28; par. Matt 5:43-48)
Repentance & Forgiveness: "If your brother sins, rebuke him; and if he repents, forgive him. And if he wrongs you seven times in one day and returns to you seven times saying, 'I am sorry,' you should forgive him." (Luke 17:3-4; cf. Matt 18:15-22)
Judging Others: "Stop judging, that you may not be judged. For as you judge, so will you be judged, and the measure with which you measure will be measured out to you." (Matt 7:1-2; par. Luke 6:37)
Faith & Prayer: "Have faith in God... Therefore I tell you, all that you ask for in prayer, believe that you will receive it and it shall be yours." (Mark 11:22+24; cf. Matt 17:20; 21:21; John 14:1)
Jesus' Destiny: "The Son of Man must suffer greatly and be rejected by the elders, the chief priests, and the scribes, and be killed, and rise after three days." (Mark 8:31; cf. Mark 9:31; 10:33-34; par. Matt 16:21-27; Luke 9:22-26)
Jesus' Identity: "Again the high priest asked him and said to him, 'Are you the Messiah, the son of the Blessed One?' Then Jesus answered, 'I am; and you will see the Son of Man seated at the right hand of the Power and coming with the clouds of heaven.' " (Mark 14:61-62; cf. Matt 26:63-64)
Jesus and the Father: "After Jesus was baptized... a voice came from the heavens, saying, 'This is my beloved Son, with whom I am well pleased.' " (Matt 3:16-17; par. Mark 1:9-11 ; Luke 3:21-22); "All things have been handed over to me by my Father. No one knows who the Son is except the Father, and who the Father is except the Son and anyone to whom the Son wishes to reveal him." (Luke 10:22; par. Matt 11:27)
Jesus and the Holy Spirit: "The Spirit of the Lord is upon me, because he has anointed me to bring glad tidings to the poor. He has sent me to proclaim liberty to captives and recovery of sight to the blind, to let the oppressed go free, and to proclaim a year acceptable to the Lord." (Luke 4:18-19, citing Isa 61:1-2)

A Compilation of the Apostles' Preaching:

The apostles preach some or most of the following points in various speeches recorded in the Acts of the Apostles (see Acts 2:14-41; 3:12-26; 4:8-12; 5:29-32; 7:2-53; 8:26-38; 10:35-49; 13:16-41; 16:30-34; 17:22-34; 19:1-7; 20:17-35; 22:1-21; 23:1-6; 24:10-21; 26:1-23; 28:23-28):

Jesus of Nazareth is the Messiah, the Christ, sent by God,
as promised in the Scriptures, as foretold by the prophets,
for the forgiveness of sins, for the salvation of the world;
He was rejected by the people, condemned by the authorities;
he suffered, was crucified, died, and was buried.
Yet God exalted him on high, raised him up to new life;
and he will one day return to us in glory.
In response, people must repent, believe, be baptized,
receive the Holy Spirit and join the community of believers.

Creedal Statements in the New Testament:

Matt 16:16 - Simon Peter said in reply, "You are the Messiah, the Son of the living God." (cf. Mark 8:29; Luke 9:20)
John 1:49 - Nathanael answered him, "Rabbi, you are the Son of God; you are the King of Israel."
John 6:68-69 - Simon Peter answered him, "Master, to whom shall we go? You have the words of eternal life. We have come to believe and are convinced that you are the Holy One of God."
John 11:25-27 - Jesus told her, "I am the resurrection and the life; whoever believes in me, even if he dies, will live, and everyone who lives and believes in me will never die. Do you believe this?" She said to him, "Yes, Lord. I have come to believe that you are the Messiah, the Son of God, the one who is coming into the world."
John 20:28 - Thomas answered and said to him, "My Lord and my God!"
John 20:30-31 - Now Jesus did many other signs in the presence of his disciples that are not written in this book. But these are written that you may believe that Jesus is the Messiah, the Son of God, and that through this belief you may have life in his name.
Rom 1:3-4 - the gospel about his Son, descended from David according to the flesh, but established as Son of God in power according to the spirit of holiness through resurrection from the dead, Jesus Christ our Lord.
Rom 10:9 - for, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved.
1 Cor 8:6 - yet for us there is one God, the Father, from whom all things are and for whom we exist, and one Lord, Jesus Christ, through whom all things are and through whom we exist.
1 Cor 12:3 - Therefore, I tell you that nobody speaking by the spirit of God says, "Jesus be accursed." And no one can say, "Jesus is Lord," except by the holy Spirit.
1 Cor 15:3-5 - For I handed on to you as of first importance what I also received: that Christ died for our sins in accordance with the scriptures; that he was buried; that he was raised on the third day in accordance with the scriptures; that he appeared to Kephas, then to the Twelve.
2 Cor 13:13 - The grace of the Lord Jesus Christ and the love of God and the fellowship of the holy Spirit be with all of you.
Phil 2:5-11 - Have among yourselves the same attitude that is also yours in Christ Jesus, Who, though he was in the form of God, did not regard equality with God something to be grasped. Rather, he emptied himself, taking the form of a slave, coming in human likeness; and found human in appearance, he humbled himself, becoming obedient to death, even death on a cross. Because of this, God greatly exalted him and bestowed on him the name that is above every name, that at the name of Jesus every knee should bend, of those in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father.
1 Thess 4:14-17 - For if we believe that Jesus died and rose, so too will God, through Jesus, bring with him those who have fallen asleep. Indeed, we tell you this, on the word of the Lord, that we who are alive, who are left until the coming of the Lord, will surely not precede those who have fallen asleep. For the Lord himself, with a word of command, with the voice of an archangel and with the trumpet of God, will come down from heaven, and the dead in Christ will rise first. Then we who are alive, who are left, will be caught up together with them in the clouds to meet the Lord in the air. Thus we shall always be with the Lord.
1 Tim 2:5-6a - For there is one God. There is also one mediator between God and the human race, Christ Jesus, himself human, who gave himself as ransom for all.
1 Tim 3:16 - Undeniably great is the mystery of devotion, Who was manifested in the flesh, vindicated in the spirit, seen by angels, proclaimed to the Gentiles, believed in throughout the world, taken up in glory.
Heb 6:1-2 - Therefore, let us leave behind the basic teaching about Christ and advance to maturity, without laying the foundation all over again: repentance from dead works and faith in God, instruction about baptisms and laying on of hands, resurrection of the dead and eternal judgment.
1 John 4:2 - This is how you can know the Spirit of God: every spirit that acknowledges Jesus Christ come in the flesh belongs to God.