History tells us that during World War II, Hitler had an interesting strategy for attacking the various European nations: he always did it on a weekend. Hitler knew the various parliaments would not be in session, making it more difficult to react swiftly to an invasion.
In the same way, the Devil will wait for an opportune moment, that decisive time to attack. It may be when our guard is down, when we aren't expecting it. It may even come when we think we are the strongest, when we think, I'm doing pretty well spiritually. I think I'm really growing. Everything is going great.
Often weaker believers are less vulnerable than stronger ones because weaker believers recognize their frailty and vulnerability. The Bible says, "Let him who thinks he stands take heed lest he fall."
Perhaps recently you have experienced a great blessing in your life. That blessing may involve your family, your career, your ministry, or your personal walk with God. But the enemy wants to rob you of it. Remember, when Jesus was baptized in the Jordan River, the Holy Spirit came upon Him in the form of a dove, and a voice was heard from Heaven from the Father, saying, "You are My beloved Son; in You I am well pleased" (Luke 3:22). The Bible says that after this, Jesus was tempted by the Devil in the wilderness (see Luke 4:1-14).
Remember, it isn't a sin to be tempted. Even Jesus was tempted. The sin takes place only when we give in to that temptation, when we open the door to it and entertain it. That is why we should flee temptation and never leave a forwarding address.