Faith without doubt is based upon God's promises. The Christian life begins with the promise of God for all who believe. The Bible says, Believe on the Lord Jesus Christ, and you will be saved (Acts 16:31). The believer in the Lord Jesus Christ is saved according to the promise of God. Faith is simply trusting what God promised. Salvation is by faith in Christ, based upon God's sure promises.
Faith without is trusting and relying upon God's promises in the Bible. Trying to live the Christian life without the promises of God becomes confusing and doubtful. The victorious Christian life is based upon what God promises in Christ that cannot fail. That's because God who has promised cannot lie (Titus 1:2).
Faith without doubt is in God's promises through our Lord Jesus Christ. For all the promises of God in Him are Yes and in Him Amen, to the glory of God through us (2 Corinthians 1:20). God keeps His promises through us as believers to the praise of His glory. Prayer based upon what God promises you is answered with Yes and in Him Amen. Trusting what God promises in your Bible is the solid foundation for all who live by faith.
Faith without doubt is standing on God's promises. That's faith with confidence in the promises of God in Christ. They are Yes and Amen in Christ. Don't let doubt rob you of victory unto the glory of God. Pray the promises of God. The answer is Yes and Amen in Christ. Your faith is sure, because what God promises you cannot fail.