Do everything without grumbling or arguing, so that you may be blameless and pure, children of God without blemish though you live in a crooked and perverse society, in which you shine as lights in the world by holding on to the word of life so that on the day of Christ I will have a reason to boast that I did not run in vain nor labor in vain. – Philippians 2:14-16
Doing all things without grumbling is a means to become blameless and shine as lights in the world(verse 15), and holding onto the word of life is also a means to that same end(verse 16). Which I think are not two means but one. You become grumble free by holding onto the word of life and as a result be blameless and shine.
I think the word of life is the gospel. Here are my arguments from Paul and Christ for saying that:
Now the noun form of the greek verb for grumbling and the greek verb for murmur immediately call to mind the murmuring of the Israelites in the wilderness (Exod. 15:24; 16:2, 7–9, 12; note also John 6:41, 43, which has Exodus 16 as its background, and cf. Paul’s explicit reference in 1 Cor. 10:10). Any doubts that this is the setting that Paul has in mind here are removed when we compare Philippians 2:15 with Deut. 32:5 LXX, where the Israelites are described as “spotted children, a crooked and perverse generation.” It is most interesting, and surely not a coincidence, that the other passage where Paul uses the experience of the Israelites in the wilderness as an example to motivate proper Christian behavior is 1 Corinthians 10:1–13, that is, when writing to a church plagued by division. One may object that the murmuring of the Israelites was directed at God. Yet here precisely we may find an important clue to the nature of the Philippians’ weakness. The complaining of the Israelites was indeed directed at God, but in the person of his representative Moses. Is it not likely—or even inevitable—that the disputes within the Philippian community involved complaints against the appointed leaders? We may point to Philippians 2:29 as a possible hint that the church leaders in Philippi were not being treated with full respect.
This means that like the Israelites, if the Philippians will not stop their murmuring against God and his appointed leaders, they will prove themselves to be spotted children who belongs to this crooked generation. Hence the leaders have labored in vain and would not have reasons to boast.
Boast about your perseverance(2 Thessalonians 1:4)
To put it another way, how can we complete their joy(Philippians 2:1-3)? Do all things without grumbling by holding our gaze to the word of life(a manner of life that shows the worth of the gospel), so be proven blameless and as a result become our pastors’ reason for boasting at the coming of Christ, that they did not labor in vain nor that we believed in vain.