"Jesus spoke these things; and lifting up His eyes to heaven, He said, "Father, the hour has come; glorify Your Son, that the Son may glorify You, even as You gave Him authority over all flesh, that to all whom You have given Him, He may give eternal life." (Jn 1:1-2 [NASB]).
Jesus has all authority in heaven and on earth. He has promised eternal life to His sheep and said, "My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they will never perish; and no one will 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." (Jn 10:27-30 [NASB]).
Therefore, His sheep are all those who hear the gospel, believe that it is true, sell/get rid of whatever is in the way and come follow Jesus by faith. God saves whom He wills. If you believe and follow Jesus then God has forgiven all your sin; past, present, and future sin.
The English words (by and through) are very close but are not the same. Every day His sheep follow Him by faith; that is the way we follow Him, because we cannot see Him or touch Him. But we know that He is and His promises are true.
While we do follow Him by faith, we were not saved by faith, but through faith. You see we were actually saved by the grace of God. God forgave our sin and to all whom He has forgiven He gives them faith in His Son Jesus.
God is righteous, holy, and just; because of this He cannot overlook sin. In order for Him to forgive sin there must be a payment, Jesus paid that price on the cross in full. So whomever God chooses to forgive by His own grace He gives them faith in His Son Jesus.
So take heart brothers and sisters, if you believe and are following Jesus it is because God has forgiven your sin in full. If then God did it all, where then is boasting? God is the author and the finisher of your salvation. Give Him all the glory, thank Him today and every day.