Chapter 18.1

i. Although hypocrites, and other unregenerate men, may vainly deceive themselves with false hopes and carnal presumptions: of being in the favor of God and estate of salvation; which hope of theirs shall perish: yet such as truly believe in the Lord Jesus, and love him in sincerity, endeavoring to walk in all good conscience before him, may in this life be certainly assured that they are in a state of grace, and may rejoice in the hope of the glory of God: which hope shall never make them ashamed.
There is such a thing as false assurance of Christian faith. We know this because Jesus warned that not everyone who says to him, "Lord, Lord" will enter the kingdom of heaven (Matt. 7:21). In the end, some people who think they are saved will turn out to be lost forever.  

Yet we should not let this sad reality keep us from knowing that there is also such a thing as the true assurance of faith. It is possible (as well as desirable) for believers to know that they are believers--for Christians to be sure of their relationship to Christ, and thus to be certain of their salvation.

The Confession offers two clear indicators of saving faith. One is sincere love for Christ. It is characteristic of true Christians to have genuine affection for Jesus.This love may be expressed through heartfelt worship, active service, or in other ways. But however it is expressed, the believer's love testifies to the believer's faith.

A second mark of saving faith is a good conscience before God. A clear conscience comes from leading a holy life. Such holiness, in turn, is produced by genuine trust in Christ, because good works always come from true faith. Genuine saving faith, makes itself evident in a sincere love for Jesus and a clear conscience before God. 

This is not to say that every believer always has full assurance of God's saving grace, The word "may" (in the phrase "may in this life be certainly assured") holds out the hope of assurance without guaranteeing its constant presence in the Christian life. Some believers--we may infer--are sometimes also doubters.  
Dr. Philip Ryken is the president of Wheaton College and the author of numerous books, the most recent of which is Loving the Way Jesus Loves (Crossway, 2012).