But I fear, lest somehow, as the serpent deceived Eve by his craftiness, so your minds may be corrupted from the simplicity that is in Christ. (NKJV)

Here is a commentary by Albert Barnes, which explains the so important “simplicity that is in Christ”:

(…)

Corrupted from the simplicity that is in Christ.

(1.) From simple and single-hearted devotedness to him–from pure and unmixed attachment to him. The fear was that their affections would be fixed on other objects, and that the singleness and unity of their devotedness to him would be destroyed.

(2.) From his pure doctrines. By the admixture of philosophy, by the opinions of the world, there was danger that their minds should be turned away from their hold on the simple truths which Christ had taught.

(3.) From that simplicity of mind and heart; that, childlike candour and docility; that freedom from all guile, dishonesty, and deception, which so eminently characterized the Redeemer. Christ had a single.aim; was free from all guile; was purely honest; never made use of any improper arts; never resorted to false appearances, and never deceived. His followers should, in like manner, be artless and guileless. There should be no mere cunning, no trick, no craft in advancing their purposes. There should be nothing but honesty and truth in all that they say. Paul was afraid that they would lose this beautiful simplicity and artlessness of character and manner; and that they would insensibly be led to adopt the maxims of mere cunning, of policy, of expediency, of seductive arts, which prevailed so much in the world–a danger which was imminent among the shrewd and cunning people of Greece, but which is confined to no time and no place. Christians should be more guileless than even children are; as pure and free from trick, and from art and cunning, as was the Redeemer himself.

(4.) From the simplicity in worship which the Lord Jesus commended and required. The worship which the Redeemer designed to establish was simple, unostentatious, and pure –strongly in contrast with the gorgeousness and corruption of the pagan worship, and even with the imposing splendour of the Jewish temple-service. He intended that it should be adapted to all lands, and such as could be offered by all classes of men–a pure worship, claiming first the homage of the heart, and then such simple external expressions as should best exhibit the homage of the heart. How easily might this be corrupted! What temptations were there to attempt to corrupt it by those who had been accustomed to the magnificence of the temple-service, and who would suppose that the religion of the Messiah could not be less gorgeous than that which was designed to shadow forth his coming; and by those who had been accustomed to the splendid rites of the pagan worship, and who would suppose that the true religion ought not to be less costly and splendid than the false religion had been! If so much expense had been lavished on false religions, how natural to suppose that equal costliness at least should be bestowed on the true religion! Accordingly, the history of the church, for a considerable part of its existence, has been little more than a record of the various forms in which the simple worship, instituted by the Redeemer, has been corrupted, until all that was gorgeous in pagan ceremonies, and splendid in the Jewish ritual, has been introduced as a part of Christian worship.

(5.) From simplicity in dress, and manner in living. The Redeemer’s dress was simple. His manner of living was simple. His requirements demand great simplicity and plainness of apparel and manner of life, 1Pe 3:3-6; 1Ti 2:9,10. Yet how much proneness is there at all times to depart from this! What a besetting sin has it been, in all ages, to the church of Christ! And how much pains should there be that the very simplicity that is in Christ should be observed by all who bear the Christian name!