"Only let us hold true to what we have attained" (Philippians 3:16)

In Philippians 3, Paul explains his goal for the Christian life in verse 10,

that I may know him and the power of his resurrection, and may share his sufferings, becoming like him in his death, that by any means possible I may attain the resurrection of the dead.

After encouraging the Philippians to do the same, Paul ends the paragraph with 3:16, "Only let us hold true to what we have attained". What is interesting is the verb, "hold true." If you look up this verb (στοιχεῖν) in a theological dictionary, you will find the following translation.

TDNT Vol. VII pp. 668
Phil. 3:16 says: "Nevertheless, whereto we have attained, (let us) remain in one and the selfsame thing."

Notice, the author of the article added the parenthetical thought "let us". The commentator did this because the word was a military term and indicated a team acting in unison. Peter O'Brien in his commentary noted the following about the verb,

Peter O'Brien
This verb, probably derived from stoikos (originally a military term for a ‘row’), initially meant ‘to stand in line, march in line’. Within the NT, where it occurs five times, stoikeo is used figuratively to mean ‘to be in line with, stand beside, hold to, agree with, follow’. (O’Brien, p. 441)

To re-iterate, the word was used as a military term to "work together". Paul in 3:16 was not encouraging the church as individuals to pursue the knowledge of Christ; Paul was encouraging the church as a whole to pursue Christ in unity. Paul laid out the overall goal, he wanted them to adopt the goal, and he encouraged them to help each other to pursue that goal.

Holiness is a group project. Sanctification is a team sport. The body of Christ should strive together to help each other to know Christ. O'Brien elaborates further on the verse.

Peter O'Brien
By using this particular verb Paul seems to be making two further points: first, if the connotation of marching in step is present, then the Philippians are being urged to move forward in unity. Recognizing that there may be differences of opinion, the apostle desires that the whole community should move forward together. He is not encouraging the presence of ‘spiritual virtuosos’, to use F. W. Beare’s expression; as Christians they need to be united in the contest in which all are engaged, working towards the same goal and ready to help one another, especially by bearing one another’s burdens. Secondly, στοιχεῖν marks a progression from attitude or orientation to that of practice. (O’Brien, p. 442)

The author of Hebrews encourages us to act in unison as well. We are responsible for one another.

Hebrews 10:24,25
And let us consider how to stir up one another to love and good works, not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near.