Commentaries:
"He who sanctifies" is Jesus Christ, and "those who are being sanctified" is us. He calls us "all of one" because we are all of one Father, and therefore of one family.
The word "brethren" indicates why the word "one" implies family. We are all brothers and sisters. If these words teach us anything, it is that Christ not only undertakes our justification but also our sanctification. Both of them are provided under the New Covenant, which He mediates.
John W. Ritenbaugh
The Covenants, Grace and Law (Part 8)
God sanctifies us through Jesus Christ and graciously justifies us by means of Christ's blood, providing us with His Son's righteousness and granting us entrance into a relationship with Him. The sanctification process writes the laws of God in our hearts and minds, making His righteousness real and practical to daily life. During this process, which requires our cooperation with Him in His purpose, we literally become conformed to the image of Jesus Christ. The overwhelming majority of Christian works come to the fore within this process as part of the preparation for God's Kingdom.
John W. Ritenbaugh
Power Belongs to God (Part Two)
Other commentary entries containing this verse:
Galatians 4:4