The central understanding of the Gospel and Paul’s understanding of it is found in the following verses:

Romans 1:16-17
16 I am not ashamed of the gospel, because it is the power of God for the salvation of everyone who believes: first for the Jew, then for the Gentile. 17 For in the gospel a righteousness from God is revealed, a righteousness that is by faith from first to last, just as it is written: "The righteous will live by faith."

Philippians 3:8-9
8 What is more, I consider everything a loss compared to the surpassing greatness of knowing Christ Jesus my Lord, for whose sake I have lost all things. I consider them rubbish, that I may gain Christ 9 and be found in him, not having a righteousness of my own that comes from the law, but that which is through faith in Christ-the righteousness that comes from God and is by faith.

The driving force of the true Gospel message is that all is of Christ. What Galatians serves as a reminder for is that we cannot give in to one bit of any other message but that all of salvation, all of righteousness, all of everything with God is now through faith in Christ and through faith in Christ alone.

Galatians 5:1, 9
5:1 It is for freedom that Christ has set us free. Stand firm, then, and do not let yourselves be burdened again by a yoke of slavery. 9 "A little yeast works through the whole batch of dough."

The warning is clear, even the smallest part of any other belief works to corrupt the truth of the Gospel. We can learn through understanding the importance of the Feast of Unleavened Bread which was to teach the Israelites the importance of trust in God and in God alone.

Exodus 12:1-20
The Passover was first and then immediately following the Passover would be the Feast of Unleavened Bread. Yeast works quickly through dough and changes its substance. The same is true of pure faith in God, false beliefs can work through it quickly and alter the substance of our beliefs!

1 Corinthians 5:6-8

In Christ we have a Passover Lamb that is above any other. (See I Peter 2:21-22) When we put our faith in Christ we have eaten the Passover! (See also John 6:47-58) After putting our faith in Christ we must never mix faith with anything but we must then live keeping the Feast of Unleavened Bread through our faith and the only way to keep our faith pure is to live in sincerity and truth! (See also John 4:24 and Hebrews 11:6)

Jesus knew of the ease of which false belief can mix with the true doctrine of faith so He warned the disciples to be careful around the teachings of different groups and of from government as well!

Matthew 16:5-6(-12), Mark 8:14-15, and Luke 12:1-12

Our safeguard against taking in any teaching but that of pure faith in Christ is to stay away from all other teachings! We can only do that if we only desire the truth of Christ.

I Peter 2:1-3