Antinomianism is the doctrine that a Spirit indwelled Christian can continually sin with reckless abandon. The fact that you tell them not to does takeaway from the fact that you believe a Christians can do this.
The entire book of 1John militates against this.
The one who continually sins, does not know God. John acknowledges that all sin, and the one who claims he doesn't in a liar. But he also obliterates the idea of antinomianism. Paul gives some instruction on this also.
Rom. 8:5 For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit, the things of the Spirit. 6 For to be carnally minded is death, but to be spiritually minded is life and peace. 7 Because the carnal mind is enmity against God; for it is not subject to the law of God, nor indeed can be. 8 So then, those who are in the flesh cannot please God.
Rom. 8:9 But you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. Now if anyone does not have the Spirit of Christ, he is not His.
You are right that we are secure. You are wrong that salvation does not cause perseverance. It change us, and when that change is not evident, it's cause for concern, and may indeed be indication of a tare rather than a wheat. Do you believe this?
And I'd be curious if you define preservation like Sproul does.