The Christian can rest in assurance if he has done his best, by the grace of God.

This probably refers to having done all things by way of preparation for the conflict. Some apply it to the Christian’s having done his best during the struggle. Trust in God never deprives one of the privilege of exerting his own God-given powers to the utmost. Although it is true that the battle will never be won without God’s armor and power, neither will it be won without the cooperation of the human with the divine (see MB 142).

Some have applied this to the last great conflict of the church with the forces of evil. The definite article lends support to this idea. Others apply the term “evil day” more generally to any day when the battle is especially fierce.

 See on v. 11. A half-armed soldier may well pay for his rashness. He goes out with a false sense of security, and the enemy is sure to seek out his unprotected parts. The Christian is vulnerable at many spots, and often that characteristic he thinks is his strongest turns out, under temptation, to be his weakest. As a chain is no stronger than its weakest link, so the Christian is no stronger than his weakest element of character. In view of the variety of foes that must be met and the various weaknesses of the flesh, nothing less than the entire armor will suffice.

The whole armor of God is available for use, and the Christian is urged to put it on. As an army must be fully equipped before it enters the field, so the Christian must be prepared with every spiritual defense before doing battle with the devil; otherwise he will suffer certain defeat.

 That is, because of the nature of the conflict described in v. 12.