The Philippian congregation was generally a wonderful group of people. Many different commentaries state that of all the groups that Paul wrote to, Phillipi was probably the best of them all. However, Paul was writing to these people with some measure of sadness because two ladies were feuding, and it was inexorably dividing the group into rival camps. In this section, the apostle is spelling out our Christian responsibility.
Notice that nowhere in the entire epistle to the Phillipians does Paul tell them, "Don't come to church." He did not say, "Split away by yourself." That is what is happening in the greater church. Paul did not say, "Just go sit in your living room." That is not an option with God. He tells us here that we have to look to and seek higher things. He says to let our conduct be worthy of the gospel that we say that we believe.
We should not be misled by the word "if" in verse 1. Paul is not stating a "maybe." He is stating an absolute fact. That word "if" is better understood as "since": "Since there are these things in you because of God's Spirit, sacrifice yourself. Make my joy complete and use them." What are we to use? Love, fellowship of the spirit, bowels and mercies. "Fulfil you my joy, that you be likeminded, having the same love, being of one accord, of one mind."
Because of God's calling, because He granted us repentance and gave us His Spirit, we have already been enabled by His Spirit to use these things to make peace, to be of one accord, to be of one mind. "The mark of the beast" can be overcome by God's Spirit in us, but we must sacrifice ourselves to use it. It is already there. Thus, Paul is saying, "Use God's love in you, and be of one mind. Quit fighting with each other to gain the upper hand. Consider the other person better than you, and serve him by looking out for his interest."
When he says, "Let this mind be in you," what he literally says in the Greek is, "Keep thinking like this." How? As Jesus Christ has already shown us. He is saying, "Don't let your mind be drawn toward what you consider to be the cause of the offense." Or, "Don't dwell upon those things."