"The most effectual knowledge for your salvation is to understand these two points: the desperate sinfulness and misery of your own natural condition, and the alone sufficiency of the grace of God in Christ for your salvation, that you may be abased as to the flesh and exalted in Christ alone. And, for the better understanding these two main points, you should learn how the first Adam was the figure of the second (Rom. 5: 14); how sin and death came upon all the natural seed of the first Adam by his disobedience in eating the forbidden fruit, and how righteousness and everlasting life come upon all the spiritual seed of the second Adam, Jesus Christ, by His obedience unto death, even the death of the cross. You also should learn the true difference between the two covenants, the old and the new, or the law and the gospel: that the former shuts us up under the guilt and power of sin, and the wrath of God and His curse, by its rigorous terms: 'Do all the commandments, and live; and, cursed are you if you do not do them, and fail in the least point'; the latter opens the gates of righteousness and life to all believers (i.e. the new covenant) by its gracious terms: 'Believe in the Lord Jesus Christ, and live,' that is, all your sins shall be forgiven, and holiness and glory shall be given to you freely by His merit and Spirit.Walter Marshall. The Gospel Mystery of Sanctification, pp 200-201
"Furthermore, you should learn the gospel principles that you are to walk by for the attainment of holiness in Christ. And here I shall mind you particularly that you would be a good proficient in Christian learning, if you get a good understanding of the sixth and seventh chapters of the apostle Paul to the Romans, where the powerful principles of sanctification are purposely treated of, and differenced from those weak and ineffectual principles, which we are most naturally prone to walk by.
"I need not particularly commend any other points of religion to your learning, for if you get the knowledge of these principal points, which I have mentioned, and improve it to a right end, which is, to live and walk by faith in Christ, your own renewed mind will cover the knowledge of all other things that appertain to life and godliness, and if in anything you be otherwise minded than is according to saving truth, God shall reveal even this to you (Phil. 3: 15).
"Yet let me caution you lest, instead of gaining Christ by your knowledge, you rather lose Him by putting your knowledge in the place of Christ, and trusting on it for your salvation.