I believe the best way to discern between a genuine believer and a "religious unbeliever," is to see what the Lord does to them when they sin. If they are living contrary to God's will for believers, and nothing happens, then they are not really children of God (Heb. 12:5-8). But, if they are rebellious children of God, you can be certain that He will not tolerate the rebellion for long! And God has warned His children, in Numbers 32:23, that no matter how hard they to hide sin, it will be brought to light; you as a child of God can "be sure your sin will find you out! You cannot hide!
God's Word says, "Am I a God at hand, saith the LORD, and not a God afar off? Can any hide himself in secret places that I shall not see him? saith the LORD. Do not I fill heaven and earth? saith the LORD" (Jer. 23:24).
Again, it says, "O LORD, Thou hast searched me, and known me. Thou knowest my downsitting and mine uprising, Thou understandest my thought afar off. Thou compassest my path and my lying down, and art acquainted with all my ways. For there is not a word in my tongue, but lo, O LORD, Thou knowest it altogether. Thou hast beset me behind and before, and laid Thine hand upon me. Such knowledge is too wonderful for me; it is high, I cannot attain unto it. Whither shall I go from Thy Spirit? Or whither shall I flee from Thy presence? If I ascend up into heaven, Thou art there: if I make my bed in hell, behold, Thou art there. If I take the wings of the morning, and dwell in the uttermost parts of the sea; even there shall Thy hand lead me, and Thy right hand shall hold me. If I say, Surely the darkness shall cover me; even the night shall be light about me. Yea, the darkness hideth not from Thee; but the night shineth as the day: the darkness and the light are both alike to Thee" (Ps. 139:1-12).
Do believers really think they can get away with sin? How stupid!