The reasons for this are as follows:
1. Iman is a term which covers words, deeds and intentions, all of which are based on one of two things: action or abstinence. Action refers to performing a deed in accordance with the instructions of Allah, which is the reality of gratitude. Abstinence, as in refraining from wrong action, requires patience. The whole of the religion of Islam is based on these two things: carrying out what Allah has commanded and refraining from that which He has prohibited.
2. Iman is based on two pillars: yaqin (conviction) and patience, which are referred to in the following ayah:
“And we appointed, from among them, leaders, giving guidance under Our command, so long as they persevered with patience and continued to have faith in Our Signs.”(Surah as-Sajadah 32:24)
It is through faith that we know the reality of Allah’s commands and prohibitions, of reward and punishment, and it is through patience that we carry out His instructions and abstain from what He has forbidden. A person can never believe in these things except through faith and that is truly a blessing from Allah. We can never carry out Allah’s commands and avoid what he has prohibited unless we have patience. Therefore patience is half of iman, and gratitude is the other half.
3. The human being has two powers, that of doing and abstaining, which control all of their behaviour. So a person will do what they like, and abstain from what they dislike. The whole of the religion is doing and abstaining, carrying out what Allah has commanded and refraining from that which He has prohibited, and none of it can be accomplished without patience.
4. The religion of Islam is based on hope and fear, and the true believer is one who is both hopeful and fearful. Allah says:
“The used to call of Us with love and reverence, and humble themselves before Us.” (Surah al-Anbiya 21:90)
The Prophet Muhammad, peace and blessings of Allah be upon him, used to pray:
“O Allah, I have surrendered my soul to You, and turned my face to You. My (own) affair I commit to Allah and I seek your protection, in hope and in fear of You” (Recorded in al-Bukhari)
So the believer is the one who is both hopeful and fearful, but these two emotions must be based on patience and gratitude: fear should make the believer patience, and hope should lead to gratitude.
5. Any action done by the human being is either harmful or beneficial in both this world and the next, or it is beneficial in one world and harmful in the other. The best course of action is for us to do what is beneficial in both this life and the Hereafter, and abstain from what is harmful in this life and the Hereafter. This is the reality of iman: to do what is best for oneself, and that is gratitude; and to abstain from that which harms oneself and that is patience.
6. As human beings, we are always in a situation where we have to carry out instructions from Allah, or avoid something which Allah has prohibited, or accept something which Allah has decreed. In all cases we must face the situation with patience and gratitude.
7. We are constantly pulled in two opposing directions. Should we follow our desires and pleasures, or should we answer the call of Allah and the Hereafter? Going against the call of whims and desires is patience, and responding to the call of Allah and the Hereafter is gratitude.
8. Religion is based on two principles: determination and perseverance (patience) which are referred to in the dua of the Prophet, peace and blessings of Allah be upon him: “O Allah, I ask You for perseverance in all my affairs, and ask You for the determination to stay on the straight and narrow path.”
9. Religion is also based on truth (haqq) and patience, which is referred to in the ayah:
“…and they (join together) in the mutual teaching of truth, and of patience and constancy.” (Surah al-‘Asr 103:3)
We are required to work according to truth, both by ourselves and with others, which is the reality of gratitude, but we cannot do that without patience, therefore patience is half of iman. And Allah knows best.
Extracted from Patience and Gratitude by Ibn al-Qayyim Pages 61-63