Jesus didn't tell people to trust their own hearts. Nor did he call them to purify themselves in order to gain God's fatherly favor. Instead, his "religion" was to point us away from ourselves to trust in him. The Christian religion says, basically, that Jesus' perfect obedience and total self-sacrifice did everything necessary to restore and secure our relationship with God. That promise is received through faith, by resting in the overwhelming generosity and good nature of the God who forgives freely. The God who gives his Spirit to sinners before they are good. The Father who purifies sinful hearts only after embracing them in all their filth and weakness.
"There is therefore now no condemnation for those who are in Christ Jesus. For the law of the Spirit of life has set you free in Christ Jesus from the law of sin and death. For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, in order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit." (Rom 8:1-4)If you are in Christ, you are truly spiritual.