The purpose of suffering according to the Bahai religion

The mind and spirit of man advance when he is tried by suffering. The more the ground is ploughed the better the seed will grow, the better the harvest will be. Just as the plough furrows the earth deeply, purifying it of weeds and thistles, so suffering and tribulation free man from the petty affairs of this worldly life until he arrives at a state of complete detachment. His attitude in this world will be that of divine happiness... Through suffering he will attain to an eternal happiness which nothing can take from him.

To attain eternal happiness one must suffer. He who has reached the state of self-sacrifice has true joy. 

Thou hast written concerning the tests that have come upon thee. To the sincere ones, tests are as a gift from God, the Exalted, for a heroic person hasteneth, with the utmost joy and gladness, to the tests of a violent battlefield, but the coward is afraid and trembles and utters moaning and lamentation. Likewise, an expert student prepareth and memorizeth his lessons and exercises with the utmost effort, and in the day of examination he appeareth with infinite joy before the master. Likewise, the pure gold shineth radiantly in the fire of test. Consequently, it is made clear that for holy souls, trials are as the gift of God, the Exalted; but for weak souls they are an unexpected calamity. This test is just as thou hast written: it removeth the rust of egotism from the mirror of the heart until the Sun of Truth may shine therein. For, no veil is greater than egotism and no matter how thin that covering may be, yet it will finally veil man entirely and prevent him from receiving a portion from the eternal bounty.

As to the subject of babes and infants and weak ones who are afflicted by the hands of oppressors: This contains great wisdom and this subject is of paramount importance. In brief, for those souls there is a recompense in another world and many details are connected with this matter. For those souls that suffering is the greatest mercy of God. Verily that mercy of the Lord is far better and preferable to all the comfort of this world and the growth and development of this place of mortality. If it be the will of God, when thou shalt be present this will be explained in detail by word of mouth. 

Through suffering he will attain to an eternal happiness which nothing can take from him. To attain eternal happiness one must suffer. He who has reached the state of self-sacrifice has true joy. Temporal joy will vanish.

Abdul Baha

O Son of Man! My calamity is My providence, outwardly it is fire and vengeance, but inwardly it is light and mercy. Hasten thereunto that thou mayest become an eternal light and an immortal spirit. Bahaullah

But suffering, although an inescapable reality, can nevertheless be utilized as a means for the attainment of happiness. This is the interpretation given to it by all the prophets and saints who, in the midst of severe tests and trials, felt happy and joyous and experienced what is best and holiest in life.

In such a process of purgation, when all humanity is in the throes of dire suffering, the Bahá’ís should not hope to remain unaffected. Should we consider the beam that is in our own eye, we would immediately fine that these sufferings are also meant for ourselves, who claimed to have attained. Such world crisis is necessary to awaken us to the importance of our duty and the carrying on of our task. Suffering will increase our energy in setting before humanity the road to salvation, it will move us from our repose for we are far from doing our best in teaching the Cause and conveying the Message with which we have been entrusted.

It is only through suffering that the nobility of character can make itself manifest.

Suffering, of one kind or another, seems to be the portion of man in this world. Even the Beloved ones, the Prophets of God, have never been exempt from the ills that are to be found in our world; poverty, disease, bereavement, -they seem to be part of the polish God employs to make us finer, and enable us to reflect more of His attributes!

Shoghi Effendi

https://bahai-library.com/warwick_meaning_suffering

I want to add a few words from Krishnamurti about suffering as well:

Totally remain with the suffering. Then you will notice, if you go that far, if you are willing to observe so closely, that something totally different takes place: a mutation. That is, out of that suffering comes great passion.