This is a mitigating enactment. As known, there are many required preparations for a battle; a worrier must have a sound body and knowledge of strategies of war. However, since the Battle of Badr was the first and decisive battle, without any advanced preparation, therefore, one believer was equivalent to facing just ten disbelievers. May be In other battles and ordinary circumstances, as time passes, the ratio would be one against two.
وَاللَّهُ مَعَ الصَّابِرِينَ (And Allah is with the steadfast.): The main factor of victory is patience, and patience represents half of faith. Allah (swt) says:
وَاسْتَعِينُوا بِالصَّبْرِ وَالصَّلَاةِ
(And seek help through patience and prayer) (Al-Baqara: 45)
Patience is a foundation for victory. When a person relies on the support of Allah (swt), he will not be defeated because the All Powerful is on his side. Allah (swt) will be his Supporter, Helper and Guardian.