There are a couple unhelpful ways of thinking, or even errors, common amongst American evangelicals which presbyterians would do well to avoid.

Secondly, some appear to think that genuine Christians do not sin, or at least do not commit particularly unpleasant sins. That may not be their doctrine, but that is their assumption when they question whether a particularly heinous sinner (or just someone who one finds unpleasant) is "really a Christian." The Bible does not teach us to think or talk this way. Peter confessed Jesus as the Christ, and then committed what was probably the worst sin of his life: denying Jesus during his trial. The simple truth is that Christians are perfectly capable of not acting very Christian, but that, by itself, is not evidence they are not converted. Faith in Christ is the only thing that makes one a Christian.
I think of chapter 18.4 of our Confession of Faith: “True believers may have the assurance of their salvation divers ways shaken, diminished, and intermitted; as, by negligence in preserving of it, by falling into some special sin which wounds the conscience and grieves the Spirit; by some sudden or vehement temptation, by God's withdrawing the light of His countenance, and suffering even such as fear Him to walk in darkness and to have no light: yet are they never so utterly destitute of that seed of God, and life of faith, that love of Christ and the brethren, that sincerity of heart, and conscience of duty, out of which, by the operation of the Spirit, this assurance may, in due time, be revived; and by the which, in the meantime, they are supported from utter despair."
A related thought has occurred to me as I’ve thought about my own life. There was a period of time when I did not act like a Christian, and claimed not to believe in God. But I was baptized as an infant, and certainly did believe in God before that time. I wandered in sin, but then again professed saving faith. If I were to say I did was not really a Christian prior to that spiritual renewal, what would that say about God? That would seem to imply that he was not part of my life at all before that time, that his Spirit had done no work in my baptism despite my childhood faith, and that he did not seek after me as a shepherd after a lost sheep. It would seem to imply that I was saved because I chose to believe again, not because the Lord was more faithful to my baptism than was I. In other words, to deny that I was a Christian for some portion of my life seems to me to detract from the magnitude of the Lord’s grace in my life, and therefore to take away some of the glory which is rightfully his.