Understand these two

From The Gospel Mystery of Sanctification:

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.