Acts 10 | Cornelius, a Roman officer is told by an angel to send men to bring Peter to him. The servants go out and before they meet Peter, Peter receives a vision from the Lord. He fell into a trance and saw a blanket with animals the old law would deem as unclean, yet God told him to kill and eat them. Peter rebukes it and tells the Lord no, then defends the law in saying the Jews can't eat anything unclean. God gives him this vision three more times, then Cornelius' men arrive to take Peter to Cornelius.
Upon Peter's arrival at Cornelius' house Peter says, "you know it is against our laws for a Jewish man to enter a Gentiles home like this or to associate with you. But God has shown me that I should no longer think of anyone as impure or unclean." After sharing their testimonies for a bit, the Holy Spirit fills Cornelius and the other Gentiles around him and Peter confesses, "can anyone object to their being baptized, now that they have received the Holy Spirit just as we did?". What followed was Cornelius and all who were present got saved and baptized in the name of Jesus.
God's will for the Gentiles to be saved wasn't going to be stopped by Peter's beliefs, so God chose to free Peter's understanding. But what was keeping Peter's understanding bound? Legalism, so much so that Peter was willing to rebuke the Holy Spirit's instruction, and challenge it with the Word. I'm sure to many of us reading are see how obvious this error was. We can cite old testament examples of God's word proving that Christ also came to save the Gentiles, but don't forget who's opened your eyes to believe that.
That's the whole point, no one can see what they don't see. Peter didn't consider his own fellow apostle, who was also saved and filled with the Holy Spirit was also Gentile (Matthew 10:4). He didn't see this because all legalism is blind to truth, and Jesus never preached legalism, He preached liberty by the Holy Spirit.
This legalism kept Peter bound, but worse than that it kept an innumerable amount of Gentiles separated from hearing Peter's testimony of Jesus Christ, and by that salvation. And this is all legalism does, separate someone from the sinners Jesus died to save by giving them a reason to reject them. Pretty nasty stuff huh... so what can we do to identify legalism and avoid it? Nothing, it was the Holy Spirit himself that revealed to Peter his legalism and the same is true for us. No matter how many people testify to someone that they're under legalism, they simply won't believe it because they believe it's what God is telling them.
We need to understand the severity of this problem, a person's whole beliefs can be so wrapped up in legalism that they make it apart of their salvation. So if you are able to show them the fault of their doctrine they will begin to question their faith in God himself. The Holy Spirit himself has to reveal to a person the error of their beliefs because God is the only one who can give life. The law is death, and all it can do is kill, but our power is in Jesus who has overcome death. We need to believe in God's love setting people free from legalism and not try to force it by our wisdom because wisdom isn't eternal, God's love is, 1 Corinthians 13:8 "Love never fails. But where there are prophecies, they will cease; where there are tongues, they will be stilled; where there is knowledge, it will pass away."
It's by faith alone we are saved, delivered and given new life. So what can we do to avoid legalism? Ask the Holy Spirit to reveal it and humble yourself enough to know that you can't come to the truth without Him.