1Co 10:12 Therefore let anyone who thinks that he stands take heed lest he fall.
Peter claimed he would never deny Christ. A man after God’s own heart David, the strongest man Samson and the wisest man Solomon had a fall season. None of them ever pictured themselves in such messes. Confidence in flesh may isolate or expose you to numerous dangers (sin, compromise, false teachings etc) making you think you can beat it, only to fall. Satan is crafty and master of description, he uses flattery words to make you rely on self confidence.
This was Paul’s concern for the church at Corinth, in 2Co 11:3 he warns “But I am afraid that as the serpent deceived Eve by his cunning, your thoughts will be led astray from a sincere and pure devotion to Christ.” Peter and the disciples knowing too well of their past failures resolved to depend on the Holy Spirit and not lean on their own understanding. To stay strong, “…they devoted themselves to the apostles’ teaching and the fellowship, to the breaking of bread and the prayers“(Act 2:42)