From: John 10:27-30
Jesus and the Father are One
(Jesus said, )  "My sheep hear My voice, and I know them, and they follow Me;  and I give them eternal life, and they shall never perish, and no one shall snatch them out of My hand.  My Father, who has given them to Me, is greater than all, and no one is able to snatch them out of the Father's hand.  I and the Father are one."
26-29. Certainly faith and eternal life cannot be merited by man's own efforts: they are a gift of God. But the Lord does not deny anyone grace to believe and be saved, because He `wishes all men to be saved and to come to the knowledge of the Truth" (1 Timothy 2:4). If someone tries to avoid receiving the gift of faith, his unbelief is blameworthy. On this point St. Thomas Aquinas teaches: "I can see, thanks to the light of the sun; but if I close my eyes, I cannot see: this is no fault of the sun, it is my own fault, because by closing my eyes, I prevent the sunlight from reaching me" ("Commentary on St. John, ad loc.").
But those who do not oppose divine grace do come to believe in Jesus. They are known to and loved by Him, enter under His protection and remain faithful with the help of His grace, which is a pledge of the eternal life which the Good Shepherd will eventually give them. It is true that in this world they will have to strive and in the course of striving they will sustain wounds; but if they stay united to the Good Shepherd nothing and no one will snatch Christ's sheep from Him, because our Father, God, is stronger than the Evil One. Our hope that God will grant us final perseverance is not based on our strength but on God's mercy: this hope should always motivate us to strive to respond to grace and to be more faithful to the demands of our faith.
30. Jesus reveals that He and the Father are one in substance. Earlier He proclaimed that God was His Father, "making Himself equal with God"--which is why a number of times the Jewish authorities think of putting Him to death (cf. 5:18; 8:59). Now He speaks about the mystery of God, which is something we can know about only through Revelation. Later on He will reveal more about this mystery, particularly at the Last Supper (14:10; 17:21-22). It is something the evangelist reflects on at the very beginning of the Gospel, in the prologue (cf. John 1:1 and note).
"Listen to the Son Himself", St. Augustine invites us. "`I and the Father are one.' He did not say, `I am the Father' or `I and the Father are one [Person].' But when He says, `I and the Father are one,' notice the two words `[we are]' and `one'...For if they are one, then they are not diverse; if `[we] are', then there is both a Father and a Son" ("In Ioann. Evang.", 36, 9). Jesus reveals that He is one in substance with the Father as far as divine essence or nature is concerned, but He also reveals that the Father and the Son are distinct Persons: "We believe then in the Father who eternally begets the Son; in the Son, the Word of God, who is eternally begotten; in the Holy Spirit, the uncreated Person who proceeds from the Father and the Son as their eternal Love. Thus in the three divine Persons, "coaeternae sibi et coaequales", the life and beatitude of God perfectly One superabound and are consummated in the supreme excellence and glory proper to uncreated Being, and always `there should be venerated Unity in the Trinity and Trinity in the Unity'" ([Pope] Paul VI, "Creed of the People of God," 10).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.