There are times that Christians can cower when they rely too much on themselves. The parable of the sower shows that not every Christian has the same degree of fruit bearing. Some bear more and some bear less. When Christians start to bear less than bear more then it can be assumed that the person still has immaturity issues to deal with. When God talks people into Christianity it is the whole package. It does not only mean you no longer live wickedly but it also means being taught love for the enemy. The old nature whispers, "Hate your enemy." but the Holy Spirit says, "Love your enemy and pray for that person's conversion."
There are times that Christians can get so tempted by money or by our circumstances that they ended up teaming up with the wrong people. God talked Christians not to be unequally yoked but times come Christians can be in such foolishness. Remember that Lot entered Sodom (and was vexed throughout his life there because that city was so wicked), Samson had his Philistine women, Solomon foolishly had 999 other wives that led him astray for some time and Jehoshaphat was foolish enough to team up with Ahab's forces. While I do not deny that they were saved and evidenced by not enjoying sin but it cannot be denied that Christians can fall into the wrong alliances God talked them out of.
Can a Christian stand firm by their flesh and by themselves? Not at all! It really takes full reliance on the grace of God and not during these times. When a Christian relies on themselves then they have ignored the source of power against sin. If you are a Christian then you are an overcome but only by the grace of God. It is all about, "God give me your grace to stand firm." and not "Self, give me your power to stand firm for God."