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For someone who is baptised into Christ ‘being with’ an unbeliever is a matter of balancing two principles.
1. The first principle is that the believer should be an influence for good to the unbeliever.
It is illustrated by Christ.
Matthew 9:10 And it came to pass, as Jesus sat at meat in the house, behold, many publicans and sinners came and sat down with him and his disciples.
11 And when the Pharisees saw it, they said unto his disciples, Why eateth your Master with publicans and sinners?
12 But when Jesus heard that, he said unto them, They that be whole need not a physician, but they that are sick.
It is taught by Christ.
Matthew 5:14 Ye are the light of the world. A city that is set on an hill cannot be hid.
15 Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house.
16 Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven.
Luke 10:29 But he, willing to justify himself, said unto Jesus, And who is my neighbour?
36 Which now of these three, thinkest thou, was neighbour unto him that fell among the thieves?
37 And he said, He that shewed mercy on him. Then said Jesus unto him, Go, and do thou likewise.
Matthew 5:44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
It is taught by Paul.
Philippians 2:15 That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world;
NIV 1 Corinthians 5:9 I have written to you in my letter not to associate with sexually immoral people—
10 not at all meaning the people of this world who are immoral, or the greedy and swindlers, or idolaters. In that case you would have to leave this world.
11 But now I am writing to you that you must not associate with anyone who calls himself a brother but is sexually immoral or greedy, an idolater or a slanderer, a drunkard or a swindler. With such a man do not even eat.
Disassociation is commanded for Christians who walk disorderly with the intention of reclaiming them, not for unbelievers.
2 Thessalonians 3:6 Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us.
2 Thessalonians 3:14 And if any man obey not our word by this epistle, note that man, and have no company with him, that he may be ashamed.
2. The second principle is that the believer is to be holy and not allow an unbeliever to influence them for evil.
Proverbs 1:10 My son, if sinners entice thee, consent thou not.
15 My son, walk not thou in the way with them; refrain thy foot from their path:
Psalms 1:1 Blessed is the man that walketh not in the counsel of the ungodly, nor standeth in the way of sinners, nor sitteth in the seat of the scornful.
2 But his delight is in the law of the LORD; and in his law doth he meditate day and night.
2 Corinthians 6:14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?
17 Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you,
Ephesians 5:11 And have no fellowship with the unfruitful works of darkness, but rather reprove them.
A believer ought not to contemplate marriage to an unbeliever.
1 Corinthians 7:39 The wife is bound by the law as long as her husband liveth; but if her husband be dead, she is at liberty to be married to whom she will; only in the Lord.
Friendships of value can only be forged between people of a like mind.
Amos 3:3 Can two walk together, except they be agreed?
We will naturally gravitate towards people whose values we share.
NKJV 1Corinthians 15:33 Do not be deceived: "Evil company corrupts good habits."
Believers are not to avoid ‘being with’ unbelievers in the ‘holier than thou’ attitude of the Pharisees; they are to love as Christ loved.
At the same time they are to avoid ‘being with’ unbelievers in any way which would compromise their obedience to the commandments of Christ.
Hebrews 12:14 Follow peace with all men, and holiness, without which no man shall see the Lord:
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I hope this helps.