Centrality of our faith, refer to the vital, critical or most important part of our faith. Jesus (Yeshua) explained it as follows in John 3:14-21
Just as Moshe lifted up the serpent in the desert, so must the Son of Man be lifted up; so that everyone who trusts in him may have eternal life.
" For God so loved the world that he gave his only and unique Son, so that everyone who trusts in him may have eternal life, instead of being utterly destroyed. For God did not send the Son into the world to judge the world, but rather so that through him, the world might be saved. Those who trust in him are not judged; those who do not trust have been judged already, in that they have not trusted in the one who is God's only and unique Son.
"Now this is the judgment:the light has come into the world, but people loved the darkness rather than the light. Why? Because their actions were wicked. For everyone who does evil things hates the light and avoids it, so that his actions won't be exposed. But everyone who does what is true comes to the light, so that all may see that his actions are accomplished through God.
Jesus (Yeshua) started by explaining the critical part of our faith by drawing a comparison with the Word Elohim gave Moses in Numbers 21:7-9 ….”We sinned by speaking against ADONAI and against you. Pray to ADONAI that he rid us of these snakes." Moshe prayed for the people, and ADONAI answered Moshe:"Make a poisonous snake and put it on a pole. When anyone who has been bitten sees it, he will live." Moshe made a bronze snake and put it on the pole; if a snake had bitten someone, then, when he looked toward the bronze snake, he stayed alive.
The critical factor here is that the people who had been bitten, had to see the snake on the pole, as they trusted in the Word of Elohim spoke through His servant Moses. In like manner Jesus (Yeshua) explained that HE will be lifted up so that everyone who trust (have confidence) in Him as the promised Messiah will receive the promise of the Father, eternal life. This promise is based on the love (grace) of Elohim have for us, that we like those in the desert can receive life, instead of the death we deserve because of sin.
The snake bite in Numbers 21 lead to death, the result of sin. The snake on the pole was the gift of life, received from Elohim. As Paul said in Ephesians 2:8 For you have been delivered by grace through trusting, and even this is not your accomplishment but God's gift.
What we must understand is that the judgement of sin was death, but Elohim send with judgment the gift of life. In like manner Jesus (Yeshua) came not to judge but to save. But as with the snake in the desert, the judgment of death already exist for those who do not trust in Jesus (Yeshua) as the Son of Elohim, will be judge accordingly.
Yeshua ends His expression of the vital part of our faith with “But everyone who does what is true comes to the light, so that all may see that his actions are accomplished through God.” Confirming that transformation or the fruit of repentance is not accomplished by human effort, but accomplished through faith in God.