if we say we have fellowship with him and walk in darkness we speak falsely and do not do the truth. Now, if we walk in the light like as he exists in the light, we have fellowship with one another and the blood of Jesus cleanses us from all sin (1 John1:6-7)
Then Peter came to him and said Lord, how often should I forgive my brother when he sins against me? Up to seven times? (Matthew 18:21-22).
We can not understand God's ways sometimes, they are above human understanding. Human understanding would say this person keeps sinning against me, they are not truly repenting. God's grace which has effects of his power working in us to mature us, says turn back to the light, keep doing this if it takes seventy times times. I think also we see how we should react to others when they do us wrong. Key here in these verses is "brother" and that they "repent and ask for forgiveness." If they have no conscience in their wrong doing and continue without any repentance and asking for forgiveness then this should be a sign God is not working in them and we should not have fellowship with them.
other seeds fell among thorns and the thorns grew and choked it out (Matthew 13:7)
Jesus said the kingdom of God is like a farmer who sowed good seed, then later that night his enemy came and planted tares among the wheat, and when the the grain sprouted and produced a crop, then the tares also appeared. At this point Jesus said the roots are so intertwined you can not separate the tare from the wheat. But there is a noticeable difference: fruit. A plant is known to be mature by the fruit, a fig tree that bears no fruit or bears bad fruit is not a good tree. Jesus said to judge those who claim to be mature by their fruit.
It is important to understand there are levels of growth in Christ. John mentions a child, young person, and the mature adult. Paul even mentions infants. So you teach accordingly to each on their level, but those who claim to be mature should have visible fruit. In the parable of sowing the one who has deep roots and understanding, overcoming the thorns is the one who bears fruit. A tare is a plant that looks like wheat until it matures then it is distinguishable by its seed or fruit. When the plant is growing its roots deep, or when it is a seedling before bearing fruit, one cannot discern the wheat from the tare. So taking scriptures together I believe Jesus is telling us not to spend our time uprooting bad plants, leave it to God, but do judge those who claim to be mature by their fruit and do not have fellowship with them.
if we walk in the light like as he exists in the light, we have fellowship with one another (1 John1:6-7)
You really can't have fellowship with those who walk in darkness, "what fellowship has light with darkness?" (2 Corinthians 6:14). This is true both of fellowship with God and with those who claim to walk in the light. It is tough love to walk in truth as do the mature. "And do not fellowship together in fruitless works of darkness but instead expose it" (Ephesians 5:11). This word for expose ἐλέγχω (elenchō) speaks of exposure, like exposing the darkness to the light. Again, only a mature plant bears fruit, so it speaks of not trying to uproot a plant but not having fellowship with them in there bad fruit/works.
do not become unevenly yoked with unbelievers, for what in common does righteousness and lawlessness, either fellowship light with darkness? (2 Corinthians 6:14)
Children of God are separated from the world by a new birth of the Spirit, so foundational is to have this new birth. Then they are to be rooted and founded in faith and love (Ephesians 3:16-17; Colossians 2:7). Until then one can't really discern by fruit other than the Spirit (1 Corinthians 2:12,15-16; 1 John 2:19). Before we became children of God, before we received righteousness, we as all people were under the influence of the prince of the power of the air who energizes the sons of disobedience. To be in fellowship with the world is be in fellowship with darkness, "For all that is in the world—the lust of the flesh, the lust of the eyes, and the pride of life—is not of the Father but is of the world" (1 John 2:15-17). "Do not be deceived bad associations ruin useful habits" (1 Corinthians 15:33).
... the latter from love, knowing that I am put here for the defense of the gospel...whether in pretense or in truth, Christ is preached and in this I rejoice (Philippians 1:15-18)
Paul is in prison when he wrote this. Why is he in prison? For proclaiming the gospel. So he is rejoicing because the gospel can now be proclaimed without being thrown in prison. Can we use this verse today and not expose those who proclaim the gospel for the wrong reasons? This would not fit with other scripture. False or unmature ministers will always justify bad behavior by using scripture out of context. They will not be servants but will lord over and exercise authority of people (Matthew 20:25-26). They will not love those within the church, thus walk in darkness, claiming to spread the gospel or ministering outward just to get money from people, for the wrong reasons (1 John 2:11). They will be peddlers of the gospel, as if someone must pay, if the gospel is not freely given without cost ( 2 Corinthians 2:17). Judge them by their fruit and have no fellowship with them.
If we walk in the light like as he exists in the light, we have fellowship with one another...
We personally need to stand and walk in the light, it is useful for us to turn from darkness, when we do wrong and not let the devil beat us up, but get back in the light where we are cleansed and have a good conscious before God. Walking in the light and having fellowship together with Jesus who exists the light is at the core of fellowship within the church.
To not love one another in the church is to walk in darkness. To not walk in truth is to not walk in the light. We need to be careful of who we let into our inner circle of fellowship. Often we just have acquaintances within the church, people we do not know well. The devil has planted tare there we are told. Even within our own families he can use members to hurt us. In the light is how we should walk in this world, and it is useful to have fellowship with those who walk in the light as we learn to walk as Jesus walked in this world of darkness.