1 Corinthians 3:15 Updated American Standard Version (UASV)
15 If any man’s work is burned up, he will suffer loss; but he himself will be saved, yet so as through fire.

This verse, this section is dealing with the Bible teacher and the Bible student. If the Bible teacher gives their best effort and the Bible student becomes a disciple of Christ but later falls away from the faith, does that mean the Bible teacher failed, that the Bible teacher must have somehow built (taught) with substandard materials?

WHO IS RESPONSIBLE?

No, that need not be the case. It can be but the falling away of the student does not mean that it is automatically the case. The section here is Paul emphasizing that all of us must share in the disciple-making process (i.e., building disciples). We want to build (make) the disciple well. However, God is telling us here through Paul that if we built as well as our abilities could, we should not carry the weight of the responsibility if the Bible student eventually turns away from Christ.

Note what Paul says concerning the teacher who has NOT done a GOOD JOB in this building (making) disciples: “he will suffer loss; but he himself will be saved.” (1 Cor.) If this Bible teacher (Christian who makes disciples) has a student that falls away or leaves "is burned up" (destroyed) the Bible teacher does no lose their salvation in this fiery test just because they are a poor teacher. So, God/Paul is telling us here that it is the Bible student who carries primarily responsible for his falling away or leaving. Now, this would be a different story if the Bible teacher had failed to teach, had not done the work of an evangelist, refusing to carry out the great commission. - Matthew 24:14; 28:19-20, Acts 1:8.

Hebrews 6:4-6 Updated American Standard Version (UASV)
4 For in the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit, 5 and have tasted the good word of God and the powers of the age to come, 6 and then have fallen away,[33] it is impossible to renew[34] them again to repentance, since they again crucify to themselves the Son of God and put him to public shame.

[33] Fall Away, Forsake, or Turn Away: (Gr. parapiptō) The sense of parapiptō is to fall away or forsake the truth.–Heb. 6:6.
[34] Renew, Restore, or Bring Back: (Gr. anakainizō) The sense of anakainizō is to cause change to a previous state, to start anew.–Heb. 6:6.

CHRISTIAN APOLOGETIC EVANGELISM: Seize Every Opportunity to Witness
https://christianpublishinghou....se.co/2018/02/13/chr