Are the prophets still relevant?

What shall Christians do with the Old Testament, especially the prophets? May we quote them? There seems such a lot of negative words spoken by them which may offend today’s generation?

Let us read what Paul taught his disciple, young Timothy, a Spirit-filled Christian pastor/teacher in charge of a number of churches.

“You have been taught the holy Scriptures from childhood, and they have given you the wisdom to receive the salvation that comes by trusting in Christ Jesus.
All Scripture is inspired by God and is useful to teach us what is true and to make us realize what is wrong in our lives. It corrects us when we are wrong and teaches us to do what is right.” （2 Timothy 3:15-16)

Briefly I summarize below the keys Paul charged Timothy, and all Christians.

1. Paul spoke of the Holy Scriptures, which include both the Old Testament and the New Testament. The Scriptures gave Timothy wisdom to receive salvation by believing in Christ Jesus.
2. Paul also elaborated further. He declared that “all Scripture is inspired by God”. Here he does not differentiate between Old Testament and New Testament. In Paul’s days they had only the Old Testament and letters written by the apostles themselves to the churches. He was referring to the Old Testament mainly.
3. Paul declared that “All Scripture is inspired by God.” There is no dispute about the divine authority of the Scripture, Old (before Christ), and New (during and after Christ).
3. Paul taught that Scripture is relevant to Timothy, the New Testament Christian.
4. Paul taught Timothy the relevance of Scripture: “useful to teach us what is true and to make us realize what is wrong in our lives. It corrects us when we are wrong and teaches us to do what is right.”
5. All these are in present tenses. The power of the Scripture to change lives is alive today.
6. What does Old Testament do today when quoted? The same effect as declared above.
7. Were Old Testament verses quoted after Jesus ascended into heaven? Yes. In the first sermon given by Peter after he and 119 disciples were baptized by the Holy Spirit, Peter quoted the Old Testament profusely (Acts 2:14-40). “Those who believed what Peter said were baptized and added to the church that day—about 3,000 in all.” (Acts 2:41)

Here is a list for those interested to read more.
Old Testament texts from the prophets quoted in the New Testament:
Isa 1:9 Rom 9:29
Isa 6:9-10 Matt 13:14-15, Mark 4:12, Acts 28:26-27
Isa 6:9 Luke 8:10
Isa 6:10 Joh 12:40
Isa 7:14 Matt, 1:23
Isa 8:12-13 1 Pet 3:14-15
Isa 8:17-18 Heb 2:13
Isa 9:1-2 Matt 4:15-16
Isa 11:10 Rom 15:12
Isa 13:10 Matt 24:29, Mark 13:24-25
Isa 34:4 Luke 21:26
Isa 22:13 1 Co 15:32
Isa 25:8 1 Co 15:54, Rev 7:17
Isa 26:19 Matt 11:5
Isa 35:5-6 Luke 7:22
Isa 26:20 Heb 10:37-38
Isa 28:11-12 1 Co 14:21
Isa 29:13 Matt 15:8-9, Mark 1:3, Joh 1:23
Isa 40:6-8 1 Pet 1:24-25
Isa 40:13 Rom 11:34, 1 Co 2:16
Isa 42:1-4 Matt 12:18-21
Isa 45:23 Rom 14:11
Isa 49:6 Acts 13:47
Isa 49:8 2 Co 6:2
Isa 52:5 Rom 2:24
Isa 52:7 Rom 10:15
Isa 52:11 2 Co 6:17
Isa 52:15 Rom 15:21
Isa 53:1 Joh 12:38, Rom 8:17
Isa 53:7-8 Acts 8:32-33
Isa 53:9 1 Pet 2:23
Isa 53:12 Luke 22:37
Isa 54:1 Gal 4:27
Isa 54:13 Joh 6:45
Isa 55:3 Acts 13:34
Isa 55:10 2 Co 9:10
Isa 56:7 Matt 21:13
Jer 7:11 Mark 11:17, Luke 19:46
Isa 59:7-8 Rom 3:15-17
Isa 59:20-21 Rom 11:26-27
Isa 61:1-2 Luke 4:18-19
Isa 62:11 Matt 21:5
Isa 64:3 1 Co 2:9
Isa 65:1-2 Rom10:10-21
Isa 65:17 2 Pet 3:13
Isa 66:1-2 Acts 7:49-50
Jer 5:21 Mark 8:18
Jer 9:23 1 Co 1:31, 2 Co 10:17
Jer31:15 Matt 2:18
Jer 31:31-34 Heb 8:8-12
Ezk 11:20 Rev 21:7
Ezk 37:5, 10 Rev 11:11
Dan 3:6 Matt13:42, 50
Dan 7:13 Matt 24:30, 26:64, Mark 13:26,14:62, Luke 21:27,22:69
Dan 9:27 Matt 24:15
Dan 11:31 Mark 13:14
Hos 2:1, 3 Rom 9:25-28
Hos 6:6 Matt 9:13, Matt 12:7
Hos 10:8 Luke 23:30, Rev 6:16
Hos 11:1 Matt 2:15
Hos 13:14 1 Co 15:55
Joel 3:1-5 Acts 2:17-21, Rom 10:13
Amos 5:25-27 Acts 7:42-43
Amos 9:11-12 Acts 15:16-17
Jonah 2:17 Matt 12:40
Mic 5:2 Matt 2:6
Mic 7:6 Matt 10:35-36
Hab2:3-4 Rom 1:17, Gal 3:11
Hab 1:5 Acts 13:41
Hag 2:6, 21 Heb 12:26
Zac 8:16 Eph 4:25
Zech 9:9 Joh 12:15
Zac 11:12-13 Matt 27:9-10
Zac 12:10 Joh 19:37
Zac 13:7 Matt 26:31, Mark 14:27
Mal 1:2-3 Rom 9:13
Mal 3:1 Matt 11:10, Mark 1:2,Luke 7:27
Mal 4:5-6 Matt 17:10-11

Habakkuk 2:3-4 New King James Version (NKJV)
3 For the vision is yet for an appointed time;
But at the end it will speak, and it will not lie.
Though it tarries, wait for it;
Because it will surely come,
It will not tarry.
4 “Behold the proud,
His soul is not upright in him;
But the just shall live by his faith.