"The faith which is in Christ Jesus," πιστιν εν Χριστε ‘Ιησου, is not the same as faith in Christ Jesus. Here he indicates how it exists in them. Faith reached them, and they heard of it by the mercy of Christ Jesus. Having heard, they submitted to it and received it into the heart through the action of Christ Jesus. Having received it, they place the hope of salvation in the one Lord Jesus Christ. On Him alone they rest by faith.
By suggesting these thoughts, without special refutations, he turns them away beforehand from false teachings. Above he mentioned our Lord, and here Jesus Christ. He is, he says, our Lord, and not His servants, that is, not the angels.
These names, Christ and Jesus, are signs of God's great benefactions toward us, according to the divine economy of salvation in Christ Jesus. For our sake He received anointing and became Christ. He also saved, and is Jesus, His people from their sins (cf. Matt 1:21). See St. Chrysostom and Blessed Theophylact.
The love which you have toward all the saints extends to all the saints, that is, to all Christians. It is not only toward fellow citizens, but also toward neighbors, and wherever they may be. The one name Christian is enough to call forth your love and be warmed by it.
He points to the union of the heart with all, born from the awareness and feelings of spiritual kinship with all who believe in the Lord. He praises active love. This means hospitality, almsgiving, and every kind of fellowship, the labor of love and not feeling alone.