Just because the current giant you are facing looks like one you defeated in the past, don't buy the lie that you never really won the first battle! By the strength of God's grace, you trusted the Almighty and conquered your Goliath. The first giant is dead. Satan is masquerading as your former enemy so he can slip past your faith and regain entrance into your life. Resist him.
More churches have been destroyed by the accuser of the brethren and its faultfinding than by either immorality or misuse of church funds. So prevalent is this influence in our society that, among many, faultfinding has been elevated to the status of a "ministry"!
Satan's arsenal consists of such things as fear, worry, doubt, and self-pity. Every one of these weapons robs us of peace and leaves us troubled inside. Do you want to discern where the enemy is coming against you? In the network of your relationships, wherever you do not have peace, you have war. Conversely, wherever you have victory, you have peace. When Satan hurls his darts against you, the more peace you have during adversity, the more truly you are walking in Christ's victory.
The enemy of our salvation especially strives to draw our heart and mind away from God when we are about to serve Him, and endeavours to adulterously attach our heart to something irrelevant. Be always, every moment, with God, especially when you pray to Him. If you are inconstant, you will fall away from life, and will cast yourself into sorrow and straitness.
Do not be despondent when fighting against the incorporeal enemy, but even in the midst of your afflictions and oppression praise the Lord, Who has found you worthy to suffer for Him, by struggling against the subtlety of the serpent, and to be wounded for Him at every hour; for had you not lived piously, and endeavored to become united to God, the enemy would not have attacked and tormented you.
If you neglect to instruct (your children) in the way of holiness, will the devil neglect to instruct them in the way of wickedness? No; if you will not teach them to pray, he will to curse, swear, and lie; if ground be uncultivated, weeds will spring.
Satan would seem to be mannerly and reasonable; making as if he would be content with one-half of the heart, whereas God challengeth all or none: as, indeed, He hath most reason to claim all that made all. But this is nothing but a crafty fetch of Satan; for he knows that if he have any part, God will have none: so the whole falleth to his share alone.
If you are a Christian, you are at war. But our approach to spiritual warfare usually falls into one of two extremes - either we place an undue emphasis on Satan and his powers or we completely ignore the existence of a personal enemy.