"Secret Prayer Family prayer and public prayer have their place; but it is secret communion with God that sustains the soul- life. It was in the mount with God that Moses beheld the pattern of that wonderful building which was to be the abiding-place of His glory. It is in the mount with God—the secret place of communion—that we are to contemplate His glorious ideal for humanity. Thus we shall be enabled so to fashion our character-building that to us may be fulfilled the promise, “I will dwell in them, and walk in them; and I will be their God, and they shall be My people.” [ 2 Corinthians 6:16 .] GW 254"
"And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people."
Ellen White directly references 2 Corinthians 6:16 in this passage.
"Christ's ministers must watch unto prayer. They may come with boldness to the throne of grace, lifting up holy hands without wrath or doubting. In faith they may supplicate the Father in heaven for wisdom and grace, that they may know how to work, how to deal with minds. GW 254.3"
"I will therefore that men pray every where, lifting up holy hands, without wrath and doubting."
EGW directly alludes to 1 Timothy 2:8, using the phrase 'lifting up holy hands without wrath or doubting' in the context of prayer.
"Prayer is the breath of the soul. It is the secret of spiritual power. No other means of grace can be substituted, and the health of the soul be preserved. Prayer brings the heart into immediate contact with the Well-spring of life, and strengthens the sinew and muscle of the religious experience. Neglect the exercise of prayer, or engage in prayer spasmodically, now and then, as seems convenient, and you lose your hold on God. The spiritual faculties lose their vitality, the religious experience lacks health and vigor. GW 254.4"
"I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing."
While not a direct quote, the passage in EGW echoes the sentiment of John 15:5. Both emphasize the necessity of connection with God (the vine/Well-spring) for spiritual vitality and fruitfulness. The 'without me ye can do nothing' aligns with the idea that neglecting prayer leads to a loss of spiritual vitality.