Note 7 at Eph. 6:13: We have already been told to put on the WHOLE armor of God in verse 11 (see note 2 at v. 11, p. 1142). Paul tells us to do it again here, and this time gives us a reason for doing so. It is because we are fighting a spiritual battle (v. 12). Physical defenses are useless in spiritual matters. We must have God's armor to defend ourselves.

Note 8 at Eph. 6:13: These verses are saying that using the whole armor of God will enable us to stand against all the devices of the devil. Therefore, it is implied that if we fail to withstand Satan's attack, we are not using all the armor of God that is available to us.

Note 9 at Eph. 6:13: The armor of God doesn't protect us automatically; we have to use it. That's what Paul is referring to here by saying we must do all to stand.