Four things that stand alone: not just necessary, but sufficient.
Christ
Question 33 of the New City Catechism: “Should those who have faith in Christ seek their salvation through their own works or anywhere else?”
“No, they should not, as everything necessary to salvation is found in Christ.”
Faith
Does Martin Luther’s declaration that justification is by faith alone, and that our works and merit add nothing conflict with James 2:24? Read R. C. Sproul’s answer at Ligonier Ministries Blog.
Grace
“Salvation is wholly of grace, not only undeserved but undesired by us until God is pleased to awaken us to a sense of our need of it. ” Read the whole quote from John Newton at Of First Importance.
Scripture
What does it mean that scripture is sufficient? “[T]o speak of scriptural sufficiency is one way of speaking about the unique authority of Scripture in the life of the church and the believer as the authoritative and sufficient source for the principles of faith and practice.” —Carl Trueman at 9Marks Blog.