St. Nicholas Cabasilas ca. 1323-1391
What is unity of faith? A double-minded man is inconstant in all his ways (Jam. 1:8) — the double-minded man being he who is doubtful and has no certainty or stability. Such a man, wavering from one side to the other, does not go straight forward upon either road. The opposite of this unhapy state is unity, namely, that which is strong, constant, and stable. He who is steadfast in faith has definite knowledge concerning any particular matter — either that it is, or that it is not. The doubter, on the other hand, is shown by his very title — amphibolos — to waiver between the two. The unity of the faith is, then, that which is unshakable and free from all hesitation.
…Therefore, he who wishes to commend himself to God and to place himself in His keeping has need of an unshakable faith and the aid of the Holy Spirit. Nor do we commend ourselves alone to God, but each other also; for, according to the law of charity, we must seek the good of others as well as our own. (Commentary on the Divine Liturgy, 14)