God is faithful the one through whom calling you into fellowship of his son Jesus Christ... (1 Corinthians 1:9)
God is faithful to keep you to the end, blameless in the day of Christ (1 Corinthians 1:8). Paul begins this letter by writing "to the church of God that exists in Corinth sanctified in Christ Jesus, called saints together with all the ones in all places calling on the name of our Lord..." (1 Corinthians 1:2). God has sanctified you and called you saints so to be in fellowship with his son.
just as he chose us in him before creating the world to be holy, blameless in his presence, in love, he predestined us for adoption as his own... (Ephesians 1:4-5)
God chose us to be holy and blameless so that he could love us and be in our presence. God doesn't love unrighteousness therefore "in him" speaks of Christ and what God eternally purposed and hid in the beginning before he created the world. In Christ, we receive adoption as his own, redemption, and forgiveness, and are favored, and sealed with the Holy Spirit until the redemption of his own. God sealed and marked us with his Spirit until he takes possession of his own, those he called into fellowship.
What was eternally proposed in Christ had the end goal of fellowship. God wanted to have fellowship with humankind. But to have fellowship, and to be loved by a Holy God we needed to be blameless in his presence. Christ accomplished what man in sinful nature could not do, by one righteous act he brought justification and life (Romans 5:18). By Adam's one transgression death reigned; sin brought judgment and judgment brought condemnation and that death.
Jesus came from the fullness of God's grace and truth (John 1:14; John 1:17). So the gift of God's grace came and the gift that came through grace was the gift of righteousness, through justification (Romans 5:16-19). Justification is through his blood, by grace through redemption, received by faith. Thus "by grace through faith are we saved" where salvation is a free gift from grace and the gift that came through grace is justification, which brings about our righteousness. Because of him we can. In him we are called into fellowship.
now those he predestined, he called, and those he called he justified, and those justified these also glorified (Romans 8:30)
God chose you and called you into the fellowship of his Son and if we have fellowship with his Son we also have fellowship with the Father (1 John 1:3). So "those he predestined, he called," (a Holy God purposed in Christ a plan for the fullness of time so that those chosen would be holy, blameless and loved in his presence, thus called into fellowship) "Those he called he justified" so to be holy, blameless, and loved in his presence God purposed in Christ grace and the gift in grace justification, righteousness, and life.
now you are from him in Christ Jesus, (in whom exists concealed all the treasures of wisdom and knowledge (Colossians 2:3)) who became our wisdom from God and these; righteousness and sanctification, and redemption (1 Corinthians 1:30)
Why did God purpose all this in Christ? because he so loved the world. Loved can be defined in many ways but one is to say one actively desires and takes pleasure in the companionship of another. Companionship is defined as the fellowship that exists between companions, those who have close connections. Communion is defined as intimate fellowship. Fellowship, which is really not an easy word to define, encompasses companionship, friendship, and communion. It is like a healthy functioning family, He is our heavenly Father and we are his own, his children. He called us into fellowship and he is faithful to keep us to the end.