Several answers can be given to this question from a Christian perspective. There are at least two external opponents, and they are the devil and the world. Both of them are determined foes of God and his kingdom and often they work together against Christ and the church. Sometimes their method is that of intense persecution whereas at other times their method is temptation of Christians to compromise their allegiance and devotion to Jesus. Moreover, they don’t engage in a short-term strategy. Sometimes their attacks can feel like a war of attrition as they persist in opposing the Lord’s cause.
In order to resist them we need to wear the spiritual armour that Paul details in Ephesians 6. Yet although their blows are severe, in the end they are not against us. If believers are persecuted and martyred for the sake of Jesus, they will receive a great reward from God. If they are led astray by a combination of the world and the devil, God will recover and restore them.
In addition to the external opponents that every Christian faces, there is also an internal power that attempts to prevent Christians obtaining the blessings that Jesus has purchased for them and that God the Father has promised to them. The internal power is personal sin, which remains within a believer after conversion. Its power is weakened because the Christian is also indwelt by the Holy Spirit. Nevertheless, personal sin is so pervasive within a Christian that it affects everything he does. In this life, he never performs a totally sinless action. At a basic level, we can say that he never gives God 100% of what could have been given, and even his best actions could have been better. Yet because we can go to God for pardon, it turns out that even our sins cannot be against us permanently.
What else can be against us? One obvious factor is our human weakness. Even if we were perfect as humans we would still not be omnipotent or omniscient. The tasks given to us as members of God’s kingdom are so great that not even a gifted person such as the apostle Paul could do any of them in his own strength. Obviously we can do outward things. For example, if I was to have three meaningful conversations daily with different people about Jesus, it would take me sixty years to speak once to each person in my home town. Of course, if every Christian in it were to do so, it would only take one month or less to reach the whole town. But even if we were to manage that target annually, we still cannot convert one person by ourselves.
Another problem that could be against us is our lack of adequate insight into the situations we face. Life from one perspective is an ongoing lists of decisions and with regard to many of them we cannot see or understand all the angles. Even if we had the power, we do not have the wisdom with which to grasp all the aspects of a situation. And most situations intertwine with numerous other ones, and all we can say to God is that we do not know what to do.
And there is the enemy of death. We can see how it destroys the hopes and pleasures of countless numbers. It seems so powerful because no-one can escape from it permanently. Yet as we observed when thinking about the martyrs, death does not prevent believers from being with God. Although it is such an enemy it becomes the door for God’s people to enter his presence. Eventually it will lead to the great resurrection.
What can be against us? Obviously a wide variety of different enemies. Can they prevent us getting to the eternal glory that God has promised? No.