What is it and do you abide in him?
There are two aspects of "abiding in Christ" that I have see so far.
First, we are all members in the body of Christ.
I Corinthians 12
18 But now hath God set the members every one of them in the body, as it hath pleased him.
27 Now ye are the body of Christ, and members in particular.
12 For as the body is one, and hath many members, and all the members of that one body, being many, are one body: so also is Christ.
13 For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit.
Romans 12:5
So we, being many, are one body
in Christ, and every one members one of another.
1 Corinthians 1:30
But of him are ye
in Christ Jesus, who of God is made unto us wisdom, and righteousness, and sanctification, and redemption:
The second aspect is our response to God's call.
Are we doing what God wants us to be doing?
Are into doing our will or are we into God's will?
Are we in ourselves or in Christ?
this refers to our fellowship in Christ.
II Cor 5
17 Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.
Romans 8:1
There is therefore now no condemnation to them which are
in Christ Jesus, who walk not after the flesh, but after the Spirit.
Romans 8:2
For the law of the Spirit of life
in Christ Jesus hath made me free from the law of sin and death.
These two verses seem to have elements of both
Romans 9:1
I say the truth
in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost,
Romans 16:3
Greet Priscilla and Aquila my helpers
in Christ Jesus:
Romans 16:7
Salute Andronicus and Junia, my kinsmen, and my fellow-prisoners, who are of note among the apostles, who also were
in Christ before me.
Romans 16:9
Salute Urbane, our helper
in Christ, and Stachys my beloved.
Romans 16:10
Salute Apelles approved
in Christ. Salute them which are of Aristobulus' household.
1 Corinthians 4:17
For this cause have I sent unto you Timotheus, who is my beloved son, and faithful in the Lord, who shall bring you into remembrance of my ways which be
in Christ, as I teach every where in every church.
2 Corinthians 1:21
Now he which stablisheth us with you
in Christ, and hath anointed us, is God;
2 Corinthians 2:14
Now thanks be unto God, which always causeth us to triumph
in Christ, and maketh manifest the savour of his knowledge by us in every place.
2 Corinthians 11:3
But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is
in Christ.
2 Corinthians 12:2
I knew a man
in Christ above fourteen years ago, (whether in the body, I cannot tell; or whether out of the body, I cannot tell: God knoweth

such an one caught up to the third heaven.
2 Corinthians 12:19
Again, think ye that we excuse ourselves unto you? we speak before God
in Christ: but we do all things, dearly beloved, for your edifying.
Etc
I John 1
1 That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, of the Word of life;
2 (For the life was manifested, and we have seen it, and bear witness, and shew unto you that eternal life, which was with the Father, and was manifested unto us
3 That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.
4 And these things write we unto you, that your joy may be full.
5 This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all.
6 If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth:
7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.
8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
10 If we say that we have not sinned, we make him a liar, and his word is not in us.
I John 2
4 He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.
5 But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him.
6 He that saith he abideth in him ought himself also so to walk, even as he walked.
etc.
Just so you know, I answered both of your questions