What love is Paul talking about when he says faith works by love?
The love Paul is referring to is the love of Christ, which He poured out for us and which sent Him to the cross. Faith works by that love, not circumcision nor uncircumcision. Believers are to let the mind of Christ be in them, who made Himself of no reputation and took the form of a bond slave.
Les explains that, in Galatians 5:6, the love behind faith's working is first and foremost the love of Christ, as He poured out His love for us. It is the love of Christ that sent Him to the cross. Les then goes to Philippians 2:5 and reminds us to let this mind be in us which was also in Christ Jesus, who being in the form of God thought it not robbery to be equal with God, but made Himself of no reputation and took upon Himself the form of a servant—or, as Les prefers, a bond slave. So the faith Paul speaks of works by Christ's love, not by circumcision or uncircumcision.
For in Jesus Christ neither circumcision availeth any thing, nor uncircumcision; but faith which worketh by love.