Christ's own words demonstrate that those given to Him by the Father, His elect sheep, shall certainly come to Him in faith.
John 6:37
“All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out.”
Notice the order and certainty of Christ's words.
“All that the Father giveth me” denotes the Father's elect, those whom He has given to His Son. They belong to Christ by the Father's sovereign purpose.
“Shall come to me” denotes the certainty of their coming to Christ in faith. Christ does not say they might come, or that their coming depends upon an independent exercise of freewill. He declares that they shall come.
The Father's giving is the cause; their coming is the certain result. Their faith does not make them the Father's gift to Christ. Rather, because the Father has given them to Christ, they shall come to Him.
Christ further explains this in John 6:44:
“No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the last day.”
And again in John 6:65:
“No man can come unto me, except it were given unto him of my Father.”
So where does saving faith originate? Not in the independent ability of the unregenerate sinner, but in the gracious work of God. Philippians 1:29 declares:
“For unto you it is given in the behalf of Christ, not only to believe on him, but also to suffer for his sake.”
This is the distinction between God's sovereign work and man's response. The Father gives His sheep to Christ, the Spirit gives them life and grants faith, and they consequently come to Christ willingly.
They do not become Christ's sheep by believing; they believe because they are His sheep.