There is no need for any Christian to worry about whether his witness will be effective. Pray these three things and trust the Lord to do His work.

For the Holy Spirit to continue His work long after your witness.  If the Holy Spirit has led you with boldness to witness to someone, you can believe that He has also been ministering to that person—that is, all along the way He has been preparing that person’s heart to hear the gospel.  Bit by bit He has been liberating that one’s mind and heart from the power of Satan; He has been helping them to see their own lostness, to see that time is short, and to see God’s love.

Let me urge you to keep praying for that lost one. Pray that throughout your witness and even after you witness, that the Holy Spirit would continue His ministry…