Full assurance is not an automatic fruit of conversion, nor is it necessarily an immediate fruit. The believer may be in a state of saving grace for a long time before attaining assurance. But attaining it is not a remote possibility it is eminently attainable and surely desirable. The assurance of salvation is an enormous benefit to the Christian, yet it is also to be pursued as a duty. The [Westminster] confession alludes to the apostolic injunction to make our election and calling sure.
Sproul, R.C.; Grace Unknown: The Heart of Reformed Theology; Baker Books; Grand Rapids, MI; copyright 1997; p. 203