Jamieson, Fausset and Brown on 1 John 5:14

14. the confidence--boldness (1Jo 4:17) in prayer, which results from knowing that we have eternal life (1Jo 5:13; 1Jo 3:19, 22).

according to his will--which is the believer's will, and which is therefore no restraint to his prayers. In so far as God's will is not our will, we are not abiding in faith, and our prayers are not accepted. Alford well says, If we knew God's will thoroughly, and submitted to it heartily, it would be impossible for us to ask anything for the spirit or for the body which He should not perform; it is this ideal state which the apostle has in view. It is the Spirit who teaches us inwardly, and Himself in us asks according to the will of God.