I read recently that a true believer can not ‘lose’ their faith or have doubts; that if they did they couldn’t possibly have been saved in the first place, for the Holy Spirit would not allow such a thing.
True believers cannot lose their faith because it’s a gift from God (Ephes. 2:8-9). At the moment of salvation He assumes responsibility for keeping us and seals the Holy Spirit inside us to guarantee our salvation (2 Cor 1:21-22). Remember, it’s the shepherd’s job to keep the sheep.
But it’s natural for us to have doubts, because the devil plants them in our minds, just like he did with Eve. (“Did God really say …?”) The Holy Spirit counsels us to believe God and resist the devil but He cannot control our thoughts. Remember, He’s a counselor not a commander.
Whenever we have doubts, it’s our job to recall God’s promises and use them to repel the enemy’s attack. 2 Cor 10:3-5 calls the knowledge of God our weapons and says they have divine power. It’s one of the reasons that knowing the Scripture is so important.