What they are telling me is basically this. A thief that steals is bad, but he is forgiven, then the thief steals, again and again, knowing that he is forgiven since "we are all sinners".
But isn't that taking advantage of God?
Yes.
Sins have unpleasant consequences.
Adam's Original Sin committed through the Great Liar, Satan, who was disguised as a friendly talking serpent/snake, resulted in spiritual death for all humankind = all are sinners and are bound for hell when they die.(GENESIS.3:14-17, ROMANS.5:12) Hence, fallen Man often involuntarily sin-in-thoughts = have immoral hate, anger, lust, greed, selfishness, jealousy, pride, fears/worries, doubts, etc, in their hearts.
If then the fallen Man go and voluntarily commit sins/evil-deeds/law-breaking, eg murder, adultery/sexual immorality, stealing, lying/cheating, idolatry, blasphemy, etc, they would be cursed by God with terror and calamities = have a sad and short life on earth.(DEUT.28:15, PROVERBS.1:25, 1COR.5:5 & 11:30, 1JOHN.5:16, HEB.10:26)
Adam's Original Sin and its curse of hell-fire can be forgiven through blood atonement because it is an unintentional or involuntary sin, as per LEV.4.(LEV.17:11, HEB.9:22 & 8:10, JOHN.1:29)
Evil-deeds/sins/law-breaking cannot be forgiven because they are intentional or voluntary = the sinners/evildoers will have to suffer the curses/consequences at the hands of God or His agents, eg convicted murderers are executed.
....... Nevertheless, God/Jesus has the authority and discretion to forgive even such intentional sins and remove the curses/consequences, eg healings, exorcisms, miracles, raising of the dead, signs and wonders.(HEB.2:4)
MATTHEW.7:21-23 = I Never Knew You
21 “Not everyone who says to Me, ‘Lord, Lord,’ shall enter the kingdom of heaven, but he who does the will of My Father in heaven. 22 Many will say to Me in that day, ‘Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?’ 23 And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’
1CORINTHIANS.6:9-11 = 9 Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, 10 nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God. 11 And such were some of you. But you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus and by the Spirit of our God.