Let's have a look at the Scripture, shall we?
Hebrews 11:1 Now faith is the substance of things hoped for, the evidence of things not seen. Read the rest of Hebrews 11 for much more on faith.
And continuing in wilfull sin is out of the question because, Romans 6:15 What then? shall we sin, because we are not under the law, but under grace? God forbid. (see also Romans 6:1)
In fact, one of the most useful gauges of saving faith is the evidence of what, if anything, has happened to the life of the person. We know that the scripture says, in 2 Corinthians 5:17, Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new. So then, if the person's life does not show that the old things have passed away (this must not be taken to imply perfection) that fact is grounds for doubt about that person's status before God. Now let me be perfectly clear about this: confidently expect your life to be entirely different, but do not be surprised when you stumble, because you will. The thing that is different after salvation is that you then have the power of God to sustain you through trials. Before receiving the gift of saving faith one has only one's own resources and they are patently not up to the task.
And do not worry about loosing what God has given. See John 10:26-29.