The command to pray for all believers is given in the context of the command to be armed, and to arm each other, with the full armor of God. We need to stand together in God’s armor because we are all wrestling with the same darkness and cannot stand against it on our own and without God’s strength.

As a brief review, I started this series with Ephesians 6:18:

The first installment had as its main point that, whatever command verse 18 is tied to, we, together, are commanded to do it at all times with all attentive prayer in the Spirit for every believer in our circle, brought before us, or brought to our minds by the Spirit. We are not to omit or refuse any. Then Spirit will also be guiding other believers in my world, who will be praying for the people I missed and for me. We will cover each other’s backs. Then we, together, will be praying for all believers, as this verse anticipates.
I promised I would build the context for the verse later.
That context begins with the familiar command to be strong in God’s power by putting on the whole armor of God:

Like all of the other commands in verses 10-18, the commands in verses 10 (‘be strong, be empowered”) and 11 (“put on”) are in the plural. Modern practice likes to individualize them, as if they were singulars, which is fine, to a point–the command to all of us together is also a command to me to do my part. But it is incorrect to read them only as individualized commands. We can only truly be strong in God’s strength when we are standing together in his strength. We can only consistently stand up against the devil’s fear and deceptions when we do so together, not separately. And we each have a role in arming each other–some pieces of armor can’t be put on by the person wearing them, by themselves! That is why these commands are in the plural and not stated as “each of you be strong,” or “each of you must put on the whole armor of God.” We arm each other and stand together. We need each other.
The reason we must stand together, being together empowered by the Lord’s strength, having put his whole armor on each other, is that we are all wrestling against the same darkness:

We are fighting against spiritual enemies, all the spiritual forces that bring evil and spiritual darkness to the world. Each of us is wrestling against a whole army of evils. I am wrestling against a whole army. If I try to do it by myself, I will lose. God gives his power to us, fighting together, not separately and not in our own wisdom or plans. We need each other.
Once Paul explains this, he repeats the command to all of us–you, plural–to put on the whole armor of God. The word translated “put on,” here and in verse 11, is pretty simple. It means to put on a piece of clothing or–in the middle voice, which is used here–to be formally invested or endowed (enduo) with a piece of clothing, to sink into a piece of clothing (en + duno), to allow a piece of clothing to be put on, or to participate or cooperate in having it put on. We don’t give ourselves the armor–it is the armor of God and is given to us. We also don’t put all of this armor on ourselves–God, and other believers, must invest us with some of it. But, when it is all put on, we together have God’s panoply (panoplia), all of his strength, everything he has, available to us to fight our enemy.
God gives all of this to us so that, when the evil day, the day of fiercest battle, comes, we will have the power, having done everything God empowers us to do, to withstand all of the devil’s schemes and come out of the battle still standing. We will win. Together.