25 For such an exalted task, the future missionary is to be prepared by a special spiritual and moral training. For he must have the spirit of initiative in beginning, as well as that of constancy in carrying through what he has begun; he must be persevering in difficulties, patient and strong of heart in bearing with solitude, fatigue, and fruitless labor. He will encounter men with an open mind and a wide heart; he will gladly take up the duties which are entrusted to him; he will with a noble spirit adapt himself to the people's foreign way of doing things and to changing circumstances; while in the spirit of harmony and mutual charity, he will cooperate with his brethren and all who dedicate themselves to the same task, so that together with the faithful, they will be one heart and one soul (cf. Acts 2:42; 4:32) in imitation of the apostolic community. These habits of mind should be earnestly exercised already in his time of training; they should be cultivated, and should be uplifted and nourished by the spiritual life. Imbued with a living faith and a hope that never fails, the missionary should be a man of prayer. Let him have an ardent spirit of power and of love and of prudence (cf. 2 Tim. 1:7). Let him learn to be self - sufficing in whatever circumstances (Phil. 4:11); always bearing about in himself the dying of Jesus, so that the life of Jesus may work in those to whom he is sent (2 Cor. 4:10ff.), out of zeal of souls, let him gladly spend all and be spent himself for souls (cf. 2 Cor. 12:15ff.), so that "by the daily practice of his duty he may grow in the love of God and neighbor." Thus obedient to the will of the Father together with Christ, he will continue His mission under the hierarchical authority of the Church.
Source: Ad Gentes (Vatican.va)