Start with Jesus' words in John 15 the importance of abiding in Him and He in us
John 15:1-27 (NKJV)
1 "I am the true vine, and My Father is the vinedresser.
2 Every branch in Me that does not bear fruit He takes away; and every branch that bears fruit He prunes, that it may bear more fruit.
3 You are already clean because of the word which I have spoken to you.
4 Abide in Me, and I in you. As the branch cannot bear fruit of itself, unless it abides in the vine, neither can you, unless you abide in Me.
5 I am the vine, you are the branches. He who abides in Me, and I in him, bears much fruit; for without Me you can do nothing.
6 If anyone does not abide in Me, he is cast out as a branch and is withered; and they gather them and throw them into the fire, and they are burned.
7 If you abide in Me, and My words abide in you, you will ask what you desire, and it shall be done for you.
8 By this My Father is glorified, that you bear much fruit; so you will be My disciples.
9 "As the Father loved Me, I also have loved you; abide in My love.
10 If you keep My commandments, you will abide in My love, just as I have kept My Father's commandments and abide in His love.
11 These things I have spoken to you, that My joy may remain in you, and that your joy may be full.
12 This is My commandment, that you love one another as I have loved you.
13 Greater love has no one than this, than to lay down one's life for his friends.
14 You are My friends if you do whatever I command you.
15 No longer do I call you servants, for a servant does not know what his master is doing; but I have called you friends, for all things that I heard from My Father I have made known to you.
16 You did not choose Me, but I chose you and appointed you that you should go and bear fruit, and that your fruit should remain, that whatever you ask the Father in My name He may give you.
17 These things I command you, that you love one another.
18 "If the world hates you, you know that it hated Me before it hated you.
19 If you were of the world, the world would love its own. Yet because you are not of the world, but I chose you out of the world, therefore the world hates you.
20 Remember the word that I said to you, 'A servant is not greater than his master.' If they persecuted Me, they will also persecute you. If they kept My word, they will keep yours also.
21 But all these things they will do to you for My name's sake, because they do not know Him who sent Me.
22 If I had not come and spoken to them, they would have no sin, but now they have no excuse for their sin.
23 He who hates Me hates My Father also.
24 If I had not done among them the works which no one else did, they would have no sin; but now they have seen and also hated both Me and My Father.
25 But this happened that the word might be fulfilled which is written in their law, 'They hated Me without a cause.'
26 "But when the Helper comes, whom I shall send to you from the Father, the Spirit of truth who proceeds from the Father, He will testify of Me.
27 And you also will bear witness, because you have been with Me from the beginning.
Then read Romans 11 below, I think this is what you were referring to brother.
Romans 11:14-27 (NKJV)
14 if by any means I may provoke to jealousy those who are my flesh and save some of them.
15 For if their being cast away is the reconciling of the world, what will their acceptance be but life from the dead?
16 For if the firstfruit is holy, the lump is also holy; and if the root is holy, so are the branches.
17 And if some of the branches were broken off (Jew), and you, being a wild olive tree (Gentile), were grafted in among them, and with them became a partaker of the root and fatness of the olive tree,
18 do not boast against the branches. But if you do boast, remember that you do not support the root, but the root supports you.
19 You will say then, "Branches were broken off that I might be grafted in."
20 Well said. Because of unbelief they were broken off, and you stand by faith. Do not be haughty, but fear.
21 For if God did not spare the natural branches, He may not spare you either.
22 Therefore consider the goodness and severity of God: on those who fell, severity; but toward you, goodness, if you continue in His goodness.
Otherwise you also will be cut off.
23 And they (Jews) also, if they do not continue in unbelief, will be grafted in, for God is able to graft them in again.
24 For if you were cut out of the olive tree (Gentile) which is wild by nature, and were grafted contrary to nature into a cultivated olive tree, how much more will these, who are natural branches, be grafted into their own olive tree?
25 For I do not desire, brethren, that you should be ignorant of this mystery, lest you should be wise in your own opinion, that blindness in part has happened to Israel until the fullness of the Gentiles has come in.
26 And so all Israel will be saved, as it is written: "The Deliverer will come out of Zion, And He will turn away ungodliness from Jacob;
27 For this is My covenant with them, When I take away their sins."
Blessings