"Now this I say and testify in the Lord, that you must no longer walk as the Gentiles do, in the futility of their minds." (Ephesians 4:17 ESV)
"Doctrine followed by practice is the distinct characteristic of [Paul's] method... doctrine and practice are so intimately related and connected that they must never be divided; Paul cannot deal even with the most practical matters except in the light of doctrine."
"Our conduct must always arise from and be dictated by and controlled by our doctrine. In other words, the Christian life is not a code which is imposed upon us and which we do not understand."
"As it follows out of doctrine we must understand what we are doing and what we are not doing. Or, to put that still more plainly, we should never do things merely because other people are doing them; and we should not refrain from doing things simply because other people refrain from doing them. We must understand why we do them or why we refrain."
"Our conduct should always be to us something which is inevitable in view of what we believe.
"If my Christian living is not quite inevitable to me, if I am always fighting against it and struggling and trying to get out of it, and wondering why is it so hard and narrow, if I find myself rather envying the people who are still back in the world, there is something radically wrong with my Christian life. Christian conduct and behaviour should be inevitable."
[On the difference between Christianity and morality...] "Christianity is interested primarily in me; and it is interested in my conduct, not in and of itself and in terms of its social consequence; it is interested in my conduct and behaviour because of its interest in Christ, in God, in the church, in the plan of redemption, in the whole scheme of salvation, in the fact that God will, through the church, astound the principalities and powers in the heavenly places. Not conduct in itself, but conduct in terms of this vast scheme; God nullifying the effects of evil, destroying the works of the devil, and restoring and reuniting in one all things in Christ. Christian conduct and behaviour always has a specifically Christian reference, and it is only rightly viewed in terms of the grand purpose of redemption."
"Failure in the living of the Christian life, therefore, must ultimately result from a failure somewhere or other to understand the doctrine and the truth."
~ Martyn Lloyd-Jones, highlights from "Practice Rooted in Doctrine" from Darkness and Light, An Exposition of Ephesians 4:17--5:17 (Baker Books, 1982), pp. 19-21.
Or hear the sermon: "Practice Rooted in Doctrine"