You're Asking the Wrong Question
Shortly before the siege and destruction of Jericho, we read about an unexpected Visitor Who comes to visit Joshua...
"And it came to pass, when Joshua was by Jericho, that he lifted his eyes and looked, and behold, a Man stood opposite him with His sword drawn in His hand. And Joshua went to Him and said to Him, 'Are You for us or for our adversaries?' So He said, 'No, but as Commander of the army of the Lord I have now come.' And Joshua fell on his face to the earth and worshiped, and said to Him, 'What does my Lord say to His servant?' Then the Commander of the Lord’s army said to Joshua, 'Take your sandal off your foot, for the place where you stand is holy.' And Joshua did so." (Joshua 5:13-15)
This Visitor was not a man or an angel, because Joshua falls to the earth and worships Him. (cf. Acts 14:8-18; Revelation 22:8-9) All indications point to the fact that this was none other than the Captain of our Salvation, the very Son of God! (cf. Hebrews 2:10; Revelation 19:11-16)
We are told very little about the conversation that takes place between the Commander and Joshua, but what we are told is quite significant.
Joshua asks a question: "Are You for us or for our adversaries?" Did you notice how the Commander does not answer Joshua's question? Rather, He says, "No, but as Commander of the army of the Lord I have now come." It is as if He is saying, "No, you're asking the wrong question, Joshua."
God does not choose sides. It is up to us to choose God's side! His will shall be done (Isaiah 46:9-11) and, thus, our victory hinges on whether we align ourselves with Him or choose to oppose Him. Which side are you on?
"The Son of Man will send out His angels, and they will gather out of His kingdom all things that offend, and those who practice lawlessness, and will cast them into the furnace of fire. There will be wailing and gnashing of teeth. Then the righteous will shine forth as the sun in the kingdom of their Father. He who has ears to hear, let him hear!" (Matthew 13:41-43)
"For as in Adam all die, even so in Christ all shall be made alive. But each one in his own order: Christ the first fruits, afterward those who are Christ’s at His coming. Then comes the end, when He delivers the kingdom to God the Father, when He puts an end to all rule and all authority and power. For He must reign till He has put all enemies under His feet. The last enemy that will be destroyed is death. For 'He has put all things under His feet.' But when He says 'all things are put under Him,' it is evident that He who put all things under Him is excepted. Now when all things are made subject to Him, then the Son Himself will also be subject to Him who put all things under Him, that God may be all in all." (1 Corinthians 15:22-28)
Those that God is for in Romans 8:31 are those who "love Him" and are "called according to His purpose." (vs. 28) How can we be called?
"But we are bound to give thanks to God always for you, brethren beloved by the Lord, because God from the beginning chose you for salvation through sanctification by the Spirit and belief in the truth, to which He called you by our gospel, for the obtaining of the glory of our Lord Jesus Christ." (2 Thessalonians 2:13-14)
Those who are spoken of as having been chosen for salvation are those who were called through the gospel!
We are called according to His purpose when we align ourselves with Him through obedience to the gospel of His Son, Jesus Christ! (cf. Mark 16:15-16; Hebrews 5:9; Galatians 3:26-27)
The key is found right there in the text of Romans 8:31...
"If God be for us..." (KJV)
IF GOD BEFORE US.
Do not ask, "Is God on my side?" For, you see, that is the wrong question. Instead, ask, "Am I on the Lord's side?" If the answer is "yes," then, truly, "who can be against you?"
"...thanks be to God, who gives us the victory through our Lord Jesus Christ." (1 Corinthians 15:57)